You didnt come for the whole day or just until usual classes begin?
The school could suspend half, or all, the grade or entire school, if they had reason to, like all those students did something outlined in the Departments policies for suspension.
But your school's full of shit. I can't see why they'd have the grounds to suspend anyone. If you took the whole day off, wouldn't it just count as any other absent day? It really shouldn't be treated any different. And if you didn't attend during the strike times, there shouldn't be an issue. I mean when the teachers strike, it generally implies they won't be teaching and you won't need to be at school.
If you were suspended it wouldn't stand up.