Try looking at it from the point of view of your employer - you said that you were available to do the work according to a set of terms that you discussed, and now you are wanting to change that. Most employers will make allowances for domestic situations in the short term, but expect the employee to work out a solution in the longer term. Schools are generally on the generous side, which is why many female teachers are able to combine being a Mum and running a career.
They are merely questioning your capability to do the job - the same as any other circumstances that stops an employee working - child care, domestic emergency, sickness absence - they are all reasons that prevent capability.