Security Deposit withheld.

[Mari Oip]

Ok this is the situation. Five girls signed a lease for $2150.00 per month for a five bedroom house. Each to pay $430 per month. The lease states it is for the term of 9 1/2 months to commence from 16th Aug to end 15th day of May. Which is 9 months EXACTLY but spands over 10 months because it included two half months (Aug & May). The landlord withheld 1/2 months rent from security deposit from all five girls because he stated it is their "STANDARD POLICY" to pay the entire last month of rent no matter what the date on the lease said. We all paid by the first of the month i.e.
Aug 1 for Aug 15 - Sept 15 $430
ETC. and all rent was PIF with no issues.

Can he have a "STANDARD POLICY" and just withhold the 1/2 months rent for the dates of May 16-31 when it wasnt on the lease?