What the original poster should do, I don’t know. But if the question is whether or not the high school will find out about the suspension if the infraction occurred after their application was submitted, I think it is quite possible the prep school will not. Or some will and some won’t. Some middle schools (or recommenders) might even reach out to the prep school proactively if they consider the offense to be particularly troubling --something like bullying or illegal activity. But middle school educators know that 13yos make mistakes and unless it is a pattern of behavior, I would hope that the OP’s teachers and guidance counselor would not label a child irredeemable or unworthy of high school admission based on one poor choice.
As far as I can recollect, my kids’ prep schools did not ask for any final record, transcript or even confirmation that they had graduated. It is possible that I’ve forgotten, but presumably I would have had to sign or submit some form to get the record released and I really don’t think that I did. Of course D24 graduated from eighth grade during the covid lockdown so everything was a blur that spring. Maybe I just don’t remember.
(As an aside, of course attendance matters, but as far as pulling a kid of middle school out to travel for weeks or even months goes, both D22 and D24 had classmates who did that at some point during grades 6-8 and it did not harm their admissions. Prep schools will tolerate a lot from their wealthy clientele to be honest. Or a less cynical take is that some schools value travel as a learning experience and presumably the families made arrangements with the middle school to make sure the course content was covered remotely so the child might have been missing class but still mastered the subject matter)