Even though it sounded “discrimination”, it all depends. Does your child have a Section 504 or an IEP? Any Behavioral Plan? Any accommodations about this? If not, school is not obligate to take your son to a field trip especially if they say taking him to a trip is risky and not safe for himself or for others. At this point, I would do whatever you can afford, whether to go with him or let him stay in school if you cannot take a day off. Meanwhile, you ask the school for a meeting to identify him for a Section 504 or an IEP. Then, you ask for Behavior Intervention Plans.