It's OK to ask if they are staying away from school.You need to know also if anyone from another union is coming out in sympathy. It's fine to get cover, NUT members can stand at the gate and talk to people as they come in but cannot prevent them from entering premises. I think there might be a limit on how many can congregate in a picket? If they stay away from school they will not be paid, but you will have to inform payroll they were absent without leave. This is how you will be able to afford cover. If there is a clear H&S issue you might have to close the school.