Even mildly optimized necromancy destroys everyone if given time and materials. For a great primer on how, I cannot recommend K's Necromancy Handbook highly enough.
My original answer follows, but it has a key mistake: I gave bad info about when 5th level spells come online. They are thoroughly in play here, which means your fortress is thoroughly quadratic. I'll discuss some of the options which most stand out to me after the quote.
Capping instructors at level 10 caps available spells at level 4.
That's good; 5th level spells are where necromancy goes bonkers
(lesser planar binding for nice corpses/haunt-shift shenanigans/magic
jar/etc). You still have wizard Animate Dead active, which is good
times, as well as Explosive Runes and so even a single level 10
wizard is crazy dangerous.
For example: spend a couple hundred gold worth of onyx zombifying
bats, then cast explosive runes all over them. When you need to kill
something, you and your bat army fly out to meet them, your bats
swarm, and you cast an area dispel magic on them (and fail all your
checks). They explode into 6d6 damage per explosive rune, and
everything in the area is now chunky salsa (probably including the
That's not a particularly great tactic either, just one which is
especially flashy and effective. Good tactics are going to depend
heavily on the local resources available. Primarily that means onyx
and cadavers, but as K explains "D&D does not currently support an
'available corpses by level' guideline." As such, resources are based
on DM fiat, which means tactics are based on DM fiat. You're also
going to have to make a number of decisions about how necromancy
actually works, which are not otherwise defined (the big one: what do
mindless uncontrolled undead do?).
In general though, you probably have a huge number of tiny things
patrolling the area. For aerial patrols, this probably means zombie
birds and zombie bats. They're cheap; they're effective in large
enough numbers; and they're great practice for students. You can get
them either directly through Animate Dead (which has a small cost)
or through judicious use of Fell Animate and Acid Splash. If you
need a real factory, you need something like a bat colony with a
spell-stitched badger at the bottom, hanging out and animating two
zombats every day without further interaction. The faculty are likely
also very eager to get their hands on mohrgs, which also solve the
You also have a zoologists fantasy of undead critters. Just about
everything can be animated, and just about everything that comes into
the area can be killed. If a hydra comes through, there's a zombie
hydra. If a tiger comes through, there's a skellington kitty. If a
dragon comes through, etc. Without lesser planar binding, there
actually is some limit on what you might expect to see, but still very
little. If the college has really had time to get established, they
may have breeding programs to keep a ready supply of subjects (e.g. if
you can get a breeding pair of cloakers, you have the cloaker aviary
supported by spell-stitched skellingtons casting Create Food and
Water), though getting the original breeding stock also falls
firmly into DM fiat without Lesser Planar Binding.
With more info, I could come up with better answers, but I hope this
is enough to get you started.
Fifth level spells are a fun time for necromancer wizards. Entire new vistas open up! My personal touchstones at this point would be Magic Jar and Lesser Planar Binding but it's not like "lesser" spells like Cloudkill aren't at the party as well. Lets discuss:
Lesser Planar Binding is supposed to be a spell where you summon an imp, then browbeat him into cleaning your kitchen for a while (or whatever), then he gets to go home. The thing is, this spell is part of the [Calling] subschool: unlike most summonings an actual imp actually shows up. So if you're a necromancer, you summon an imp, stab him, and then you have an imp corpse to turn into a zombie (decent AC, flight). But the only real limit is the 6HD cap on the spell, so you could do a Dark-Spellwarped-Draconic-Phrenic-Shadow Creature-Half Vampire Fleshraker Dinosaur just as easily (which might make a nice skeleton for the breakfast nook). Whatever fits your idiom, really.
I've had my eye on Kythons (from the Book Of Vile Darkness) as a contender for the most broken chassis, but haven't had a chance to try them yet.
K's handbook has some suggestions on what to look for when deciding whether to make a skeleton or zombie; I won't paste that here but you should read it. Remember, you don't actually need to do much with zombies or skeletons. If uncontrolled undead just stand around or follow their last orders, you can literally stack them in the corner. If you need to isolate them, remember that Wall of Stone came online this level as well: you can trivially build isolated cells for them, then pave over the top and use it as a basketball court. To break into your stash, you'll need a serious burrowing creature. Probably that means a zombie umber hulk standing awkwardly in a display case labelled "break glass in case of emergency".
Remember that there are clerics about, so every undead is going to be created inside a Desecrate spell (with altar), so they will all have +2 hp per HD.
Magic Jar lets you replace your feeble nerd body with an awesome one. You can use Lesser Planar Binding to find whatever one you like, or you can just hop into whatever zombie dragon you were keeping around anyway. This has a couple quirks to be aware of with respect to range between your original body, the focus, and your current body. It only check distances when someone dies or when the duration expires though, so it's very workable. Probably you want to do something like put the focus in a bird zombie, then carry the bird with your possessed body. If you die, the bird flies back. You can possess the bird if you need to. If your opponents were careless, you even get to try to possess them (leaving your demon/zombie to rampage in the meantime).
With a whole college of necromancers, you get even stupider options. For example: stick a half dozen focii and a collection of zombies inside a hollowed-out draco-zombie, and you have a mothership that rains down full-casting zombie succubi (again, adjust to suit your idiom). Every time one dies, he just grabs another body out of the ship and comes right back. Is the draco-zombie mothership actually good? No idea, but it's a great example of the crazy-go-nuts options which open up to necromancers at this level. It doesn't take any DM fiat either, this is all world-agnostic* combinations of core material.
(*okay, not entirely world-agnostic. If succubi simply don't exist in your multiverse, they can't be called and made into zombies. But anything that exists anywhere in your universe is fair game, and it's usually fair to assume that means anything in the core books.)
If you were playing 3.5, I would add Haunt Shift to this list. You're not, so I won't, but it did add a whole new layer of crazy.