In reply to RX-78:
Been living in SAT rentals for over a decade and now finally looking to buy soon. Yes, this is a bit unfair, and yes, it's very common. In my experience, it's tough to get an agent to vary at all from their standard, in-house way of doing things. If you want to do things differently, it's easier for them to just tell your son to naff off, and find another student to fill the gap. I've always lived in places where housing demand is high though, so if walking away and leaving them with an empty room is 1) actually a problem for them and 2) something you could do, then you might have more leverage to negotiate.
Other things to think about:
You could look for a private landlord. Less likely to do this, more likely to negotiate.
Look for a smaller flat, where the total rent is less and your liability is less, even if the per person rent is higher.
Look for a huge flat with rooms rented individually, not the whole flat joint and several (this type of guarantor agreement only makes sense on a joint and several tenancy).
Try to make sure the co-tenants and/or their parents are trustworthy and solvent.
Also bear in mind the agent will be looking for large incomes or guarantors for all the tenants, and although they can pursue you for the whole rent, they wouldn't do so unless they had failed to get it from the other students and parents. So if they are doing their job halfway properly it's quite a low risk. Although these agreements are common, I've never heard of a guarantor actually being pursued for anything beyond their child's part of the rent. I'd be quite interested if anyone has an example of a County Court Judgement or Sheriff Court Decree enforcing this type of agreement.
Also bear in mind that even if you don't sign a guarantor agreement covering the whole flat, your son is probably stuck with a joint and several agreement. So he will be liable for the whole rent, even if you're not. Would you let him go bankrupt rather than bail him out under those circumstances?