Send an email to the teachers and cc the principal. After 24 hours, if you haven't heard anything, call the school. You need answers. Develop a communication plan with the school that you both can agree upon.

I would email and ask his teacher. Anytime i see stuff like that on ashley i ask the teacher but usually the teacher reaches out to me first and tells me what happened

I would say to each teacher at different times but at the same day.. after school I noticed my son had marks on his arms that werent there this morning. Did you see anything he could have been doing to cause those marks?
(Assuming the answer is no) did any other teachers mention anything? ( of course the answer is no) but you will be raising awareness that everyone needs to monitor your child closely.

I hate when things like this happen because we as parents have to play the guessing game as to what happened.

I would ask in writting what the teacher thinks may have happened.
Giving him/her a chance and notice to be more alert when kiddo is in their care.