Probably because when they joined, they were a HS student. I like @CC_Mike 's solution of adding Class of XX as that will always be accurate even if they come back several years later.
ETA, can we change the other HS groups to be labeled with the same format? I just saw a post by a current HS Senior that should be labelled HS Class of 2022. Here is his post with HS Senior next to his name. Public out of CA schools that can be considered safeties! - #18 by pricklypear1