Does property management com have right to hold last month's rent

[Susan Shen]

We have a duplex rented out as student housing on a yearly lease in Philadelphia. 

Before move in, tenants are required to pay 1st, last months' rent and one month rent as security deposit. Based on contract we have with property management company, they hold the security deposit. It was vague regarding the last month's rent. My position is that the PM company has no right to hold the last month's rent, since it is rent, it belongs to the owner and owner should get paid as it is collected. What do you guys think?

The is my second question. We (the landlord) did a walk through of our property a couple of weeks ago. We notified the PM in advance and before we entered the property, the PM told us that tenants had been notified. But when we went to the first apartment, tenants told us that nobody notified them. We went into the 2nd apartment, there were no tenants there at the time. We found a device for smoking pot. 

Is it illegal to smoke pot in the apartment, is it also a fire hazard? If indeed the PM did not notify the tenants, can we complain about their use of weed? What is the legal and right thing for us to do as the landlord?

Thank you all for your input

Susan