Because they were having fun!

I think a lot of people, no matter what their background, would not mind going to school. What else are you going to do with your teens for 5 days a week? School is probably where they do their best drug business.

An also why would the principal send his daughter there instead of a nice private school
Channel Six News, they'll finger anything with a pulse!

School is a great place to sell drugs.

Got bangers like cody out of jail early,they went to school in exchange for early parole,the guard told them at the start of the movie,any trouble n they would be straight back in.
I'm running this monkey farm now frankenstein

Even gangbangers need an education.

Presumably, most of the "normal" kids, or kids with good, stable families attended either private schools or prep schools, leaving primarily the delinquents, gangbangers, and poorer kids attending the public school.s

The "normal" kids would just be other gang affiliates, just not the most violent ones. I mean, the younger brother didn't seem that dangerous, surely he was in a class with human teachers.

Most of the kids that seen in the robot teacher's rooms throughout the film are the bad kids, so it would assume that human teachers handled the normal students.
It is unlikely that there were very many "normal" kids at the school. Remember that it is in the center of a free-fire zone. Between the front door of the school and the edge of the zone there are no laws. Any normal, unarmed kid would get eaten alive within their first week at the school.
What Would Jesus Do For A Klondike Bar (WWJDFAKB)?

This film was produced in the late eighties and early ninties when gang and youth violence was increasing within our public schools. Really what this film really is a giant satire of our public school system and it was seemingly ahead of its time in some regrads. For example, kids brought guns to school in the film out in the open with police and medal detectors at the front entrance of the school. Granted kids do not bring fire arms to school out in the open like in the film, but post-Columbine public schools had to have metal detectors and zero tolerance policies to maintain order. What the film was trying to get across in 1990 was that by the turn of the century violence would escalate in our public schools that draconian measures would have to be implemted to keep order with the students. What an ironic twist of fate?
As for why the students that were gang members still attended classes, perhaps it has something to with the writers trying to say something about Reagonomics. In present day alot of governments are making it mandatory for students who come from families on welfare to attend school or else be terminated from the program. Although it was never mentioned in the film, perhaps some of these student gang bangers participated in welfare for the sake of their familiy and did the gangster stuff on the side. If you look closely at some of the dialouge you can tell that there is a middle class versus under class going on.
There are other teachers in the school, but the dialouge at the begining of the film seems to indicate to me that all of the more "unstable" students were funnelled to the android teacher's classes. Most of the kids that seen in the robot teacher's rooms throughout the film are the bad kids, so it would assume that human teachers handled the normal students.