I would disagree with Amit. Banks even if Govt Bank have to maintain secrecy of account and even RTI Act is not valid on this. Even if you get unauthroised information you cannot produce before the court. (though it can be used in cross-examination)
I fully agree with Mr Rajeev. The school management can be summoned. There is a danger that they may just sack here and depose that she is no more working with them and employ her again after 6 months. You will be left with no evidence.
I fully agree with Mr Arvind. each education al institution submits details to Distt Education Officer / Education Board University . In such report they submit more salary (as per Govt rates) than actually paid.
Even the ITO can be summoned to produce F-24 of the institution which contains details of TDS.
If you have the prospectus, you can even have the evidence of some parents or ex-parents of the school.