would they really be going to the same high school?
Probably not, although it's possible that Daniel lived close to the boundary. It all would have been part of the LA Unified School District.
Reseda was more working class (not necessarily "poor"), while Encino (where Ali and the Cobra Kais were from) was more upscale, upper middle class. The Valley is like its own separate area, while the "real" LA is on the other side of the mountains. Places like Bel Air and Beverly Hills were far more posh and wealthy than even Encino. The poorer side of LA would be more like South Central, Watts, East LA, Compton - places where the entire Cobra Kai would be wiped out in five minutes.