"I'm in Law school."
Translation:Я учусь на юридическом факультете.
"в" is not used with the departments of a university. Факульте́т, отделе́ние and ка́федра are all used with на (probably stemming from the fact that oftentimes these are formal divisions that may and will coexist inside the same building, occasionally using the same rooms).
Университе́т, институ́т, ко́лледж and шко́ла are used with в (even though a large university can have an "institute" amongst its subdivisions).