Didn't say they were spongers, just think they should contribute to local services if they are going to live here. Doesn't have to be 100%, but 8 students in a HMO generate a lot of waste, use roads and local amenities and should contribute to the police force that keep them safe.
The idea of having different rates of charge for this and any number of other things (rail fairs, prescription charges, BHA season tickets) basd on demographics seems very fair but it is very complicated and expensive to manage. The great proponent of this, wrt taxes and benefits, was the great......Gordon Brown.
Keep it simple. It requires fewer people to curate and is cheaper. And fewer cock ups. After all, do you select a dose of paracetamol based on body weight? Strictly speaking this is the most appropriate way of doing drugs. But....we don't.
Anyway, moving on....
edit I see [MENTION=600]Bry Nylon[/MENTION] has beat me to it, albeit more succinctly