As per the Right of Children to Free and Compulsory Education Act, per 30 students 1 teacher is considered as standard. Students strength above 150, 1 Head teacher is considered.
As you said for roll strength less than 150, in my opinion this may be the scenario:
i) The School will run without head teacher.
ii) All the teachers will act directly under concerned S.I.
iii) The senior most teacher may act as Teacher-in-Charge.
iv) No additional allowance is admissible.
Love for All; Hatred for None.