It was the teacher strike today, and i stayed at home when school was partially open...?

Explain to your mom that going to school today would have been a waste of everyone's time; none of your teachers were there, and you could just as easily have read a book at home. Don't lie, just have your mom write the letter saying you didn't go in because of the strike. If you get in trouble, bring your parents in and have them talk to whoever is punishing you. If that doesn't work, suck it up and serve your detention or whatever just to placate the school. Honestly, I would've done the same thing if most of my teachers were striking or otherwise off-campus; at that point, all the school is doing is just large-scale babysitting.