My child was questioned at school by a school resource officer. I wasn’t even told that it happened until afterwards. Can the statements he made to the officer be held against him?

Yes. School officials, even school resource officers are not required to contact parents prior to interviewing students nor do they read them their Miranda rights.   Anything they say can be used against them. The safest thing to tell your child, especially since you may not be present, is to ask for an attorney and say nothing. Gone are the days where kids would get in a fight at school and be dealt with at school. The kids are now arrested and prosecuted even for schoolyard fights.