June Month Not Being Counted In Notice Period Of Resignation In Private School

I am currently employed in a private school serving my notice period. I gave resignation in june so my notice period of 3 month should be over in September. But my school is saying that June is not counted in the notice period as per delhi school education act. I went through the whole act but nothing as such is mentioned there. On asking they showed me a book by a publisher in which it is mentioned under model service rules. My question is it's not a part of the Delhi school education act but under model service rules. Are these model service rules also abiding on the teachers. In my contract also only 3 months is mentioned not 3 working months.