Most landlords do not like you to sub let. You'll also find that if you sign a standard AST it'll be one of the conditions that you don't, so, if you break the terms of your tenancy agreement, you could find yourself being asked to leave - not really what you need during your degree. At the very least, the landlord is unlikely to renew your contract (which is likely to be for six months initially), which ultimately will cost you more money as you'll have to fork out again to move. I also believe that in cases like this an exchange of monies does constitute a form of contract between you and the person you sublet to (although I might be wrong there) so you can't wriggle out of this by saying there's no contract. Also, going off my own experience with lettings agents/landlords, they will not be happy if you move in non-UK residents as it's impossible to chase them for monies/damages etc. If you are a non-UK resident hoping to rent in the UK, any reputable landlord will insist on a UK based guarantor or rent up front. I mention this because it's one of the reasons why what you're doing WILL annoy your landlord.
On another point, you are risking your own deposit doing this. Remember that YOU will be held liable for any damages, bills etc etc. If you sublet to someone illegally then you've absolutely no comeback should they damage the property or run up massive bills - YOU, as the legal tenant, will have to pay.
It is up to you to sort out the council tax and the exemptions. If you claim that only you and your girlfriend are living there and this is found not to be the case, then the ultimate penalty for council tax evasion is prison. Ironically, council tax evaders are the only prisoners who still have to pay council tax in prison. If your friends are working and have a national insurance number then at some point they are going to have to give an address - do not assume that it'll never be found out that they're living with you, it will. Councils do not look kindly on people who try to evade paying council tax.
I don't mean to sound harsh here but what you're proposing to do is dishonest, and it's one of the reasons why even good tenants (students especially) are treated with such suspicion by landlords. You're really not helping yourself if you try to defraud people.