I'm not strictly disagreeing with what everyone else has said, but.....
I wonder if it is actually illegal? If she is genuinely going to rent your place and actually move into it, then she is fulfilling application criteria. There are plenty of people who buy specifically in the roads around Honeywell etc in order to get their kids in, do people who can't afford to do so, but maybe live within a mile of the school and not get places there, have the right to jump up and down that the system is unfairly stacked in favour of the have's and have nots.
Also, if a family were genuinely living in a catchment, but then at a later date decided to move and their new house wasn't in the catchment, but still in a reasonable distance, would they be expected to remove their kids? No, of course not. I'm guessing your friend doesn't live miles and miles away, as why would she want to take on a ridiculous school run when you move back.
The whole system is a bit buggered, we all know that, not enough good schools, too many kids, but I'm afraid its the law of the jungle out there, and you have to do what you can to get the best for your kids. Those of you who are throwing your hands up in horror at the unfairness of it, if you are unlucky enough not to have a good school for your kids, given the opportunity to get them into a trophy school, if it meant renting for 6 months, bet you'd all do it.
I know there's something a bit underhand about it, but I found the tone of the shock, horror responses a bit much, and I'd say go for it. If it was a friend of mine asking the same favour, I'd be saying yes.