Hello Vainnoophou88
Yes, as long as you are a school, you are welcome to use the Day Pass for schools.
If you miss the train? That should not happen with a proper planning. If you still do, you were going to lose your priority. Depending on the group size, you even might face a boarding denial if there is no sufficient space on board a service. The day pass for schools is an inexpensive offer; the string attached is that you have to make reservations in advance to allow the train operators to plan their rolling stock, to accommodate all the groups.
Kind regards,
Roland