There are so many possible causes. It could be that she is a little stressed by preschool, and that can cause a rise. It could be that the number was ok, but was on the way up when she was getting to school. It could be snacking at school, getting a bite of some other kid's snack. It could be the time of day her basal needs are different, and you see it when she is at school just because of the time.
Is she new to preschool, and this is possibly just a new thing that is upsetting her?
There are probably a lot of other possibilities I am not thinking of.
Is it always the same, every time she is in school, but not at home? Is it always the same time of day? If she was at home during that same exact time, and you did everything else the same, would her BG rise?
Possibly start with analyzing time of day, and basal rates during that time. See if she is ok during that time when at home, but not ok at school.
I would start by trying to figure out whether it is either time of day, or school.