In my state, it is not uncommon to expel one who is 18 or older for certain offenses as they can usually obtain a GED or attend adult ed programs to complete their education. I suspect this was not the youth's first offense.
School is not the place for sexual encounters. They could have probably called the police as well, so he should at least be thankful he is not facing a potential criminal rap, too.
There is likely an appeals process he can access, so he needs to exhaust all of those remedies before he has any real legal recourse. Then, of course, he has to come up with the funds for an attorney to look at the case and MAYBE do something. Chances are, however, the school year will be over before anything would get to court.
It might be best to consider adult ed or a GED at this point.
- Carl