I own a house that was rented to students for one year on a lease who have moved out. The carpets were cleaned prior to them moving in but they did not clean them when they left so they needed to be cleaned for the new tenants. Am I allowed to keep the cost of cleaning the carpets from the security deposit or is that considered normal wear and tear?

"Normal wear and tear" does not include dirt. A tenant is supposed to return the premises in the same condition they were when they moved in, except for normal wear and tear. For a carpet normal wear and tear would be worn patches in high traffic areas. Dirt and stains are not normal wear and tear. If I were in your position I would withhold money from the security deposit for cleaning. If you do this then make sure you get a detailed invoice from the company doing the cleaning and provide a copy to the former tenants.