Is it legal to wrongly mark a student truant?

It is legal. Depending on your school policy (or your coach's individual policy), you could very well have been late. Generally, you are late if you are not ready to begin cl when cl begins. Some teachers will consider you on time if you are in the room at the start of cl. Others require you to be at your seat and ready to work. If you were supposed to be in the pool at the start of cl and were not, your coach might consider you late. I know that as a teacher, I would not consider a student present if he or she were in the clroom wandering around rather than at his seat and working. However, in this case, I would contest it, especially if there were witnesses who saw you there at the start of cl and heard the coach tell you to pull off the tarps. If you were supposed to have the tarps off prior to the start of class, then you may still be out of luck, but it's worth the effort.