One problem you're going to run into with any traditional RPG is going to be size of your group. 8-10 players is usually pretty large for a cohesive RPG experience.
That said, I'd recommend three different systems. They all lack something of your requirements, but in both cases that shortcoming can be overcome with a bit of creativity.
My first recommendation would be Dungeon World. It is a D&D sort of game, where the group goes in search of adventure in a fantasy world. There are different classes, with each falling into a specific role based on their use of moves. These moves give the ability to attack a monster, heal a party member, or do other things that are somewhat specific to the needs of the party. You can act outside of the restrictions of these moves, but you will be penalized for it. The nice thing about this one is that it's pretty strictly narrative; the players narrate what the characters do, and if they have a move that would be triggered, that move just happens. It does lack specific strictures on what players can do, and with creativity they can get around limitations.
My second recommendation is Leverage. It is based on the television series of the same name, and each character takes a role in a group that pulls off heists. While it does satisfy your requirement of the characters working together, as each character is exceptional in his own area, but only mediocre in others, I hesitate on giving this a thumbs up for a few reasons. First, it's geared towards adults in themes. You could get around that with a bit of re-work however. Second, the system is a bit crunchy. It does work well with adults and is fast paced, but I think that will fall down with younger players. Third, while it is based on doing the right thing- the way that they are done isn't necessarily a good message for younger kids. Again, this can be overcome with work on your part, but the default scenario is one in which everyone is on the wrong side of the law doing good things.
My final recommendation would be Do: Pilgrims of the Flying Temple. This is also the one I'd most recommend. While it does not have set classes or roles, it does emphasize helping each other through obstacles and everyone getting an equal turn to affect the story. It's also quite geared towards younger players and is very light hearted. Again, it is a narrative game, so it helps inter-person relationship skills.
One thing that all three have in common is that they are narrative games- they are based more around the story, playing the character, and crafting a narrative rather than merely rolling the dice. I think that no matter what you choose, this would be a good direction to take, as it helps them to talk through problems with the obstacle and each other.