School attendance laws vary a lot from state to state. I would see if this is OK per you state's laws. I'm in PA and I know the school nurse can excuse an absence if a student is sick. If they do end up suspending your child, I would want Monday to count toward the suspension. I would ask the school to document what they said during the call since a phone call isn't tangible proof the school said for him to stay home & the absence will be excused. This can be done with an email:
Dear School-
I'm looking to confirm the call from (school administrator) that I received on Tuesday requesting that I keep my son, XX, home on Monday, 11/27, so the school can investigate a fight of some sort that happened on Tuesday. I was told this will an excused absence but I thought that could only happen for medical reasons. I don't want keeping him home like (school administrator) requested to turn into a truancy issue since I only have a call as proof that the absence will be excused.
If I have understood things correctly, there is no need to reply to this email.
Thank you,
There is a saying in schools: If it's not in writing, it didn't happen. This email puts things into writing.