In D&D5E, all wizards select a subclass - most of them are school "specializations", but others exist. Taking a particular school does not bar the character from casting the full range of wizard spells<sup>1</sup>, but give a few special abilities related to the type of magic they're best at.

True enough, your character's *favored* spells would be less effective against creatures with truesight. However, by the time a wizard reaches that level, there should be plenty of other spells in his arsenal. As long as you don't *neglect* other schools, your character should be fine.

<sup>1</sup><sub>In previous editions, specialist wizards lost out on entire schools of magic to make them better in their selected school.</sub>