Aaaaah, my pet hate, the Oxford Reading Tree!!!
Dd1 (9) had these at school; with ds (nearly 5, in reception) the school are doing more phonic-based work and using a reading scheme from NZ which I think is better.
My big gripe with the Oxford Reading Tree is that with the pictures it's pretty much idiot-proof, but without the pictures it's actually quite hard -- they use words like 'guitar' and 'headache' which the child stands no chance of being able to decipher. In other words, it encourages the child to look at the pictures and guess.
ORT get better further up the scheme -- at about Level 4 they introduce a set of quite gripping (for a reading scheme!) stories about a magic key that leads the children into lots of adventures.
IME how often children get their books changed depends on staff availability at the school. My ds gets a book changed twice a week, but never gets sent home with a book in the holidays. The level the child is given may or may not be appropriate to his/her reading ability: it depends on how on the ball the teacher or assistants are.
The children's actual reading level will also vary hugely my dd1 learnt to read freakishly fast, and had gone off the top of the ORT by the time she turned 5. But then again I had to go into the school every other week to throw a strop because she was so bored. Ds is much more normal can read many of the common words, but then will also unaccountably forget them, so needs quite a bit of support.
FWIW I think it's wise to assume that the person with primary responsibility for ensuring your child learns to read will be you. Get a set of books you like (check the library) and work through them. We use the same Ladybird keywords books I learnt to read from in the seventies they're very dated but I think v. good. Dr. Seuss is also good Ten Apples up on Top, Put Me in the Zoo, Hop on Pop are the easiest. It doesn't really matter as long as it's not too difficult, and just keep practising. That way you'll get a good idea of what she can actually do, and will give you a better basis for moaning to the school, if it comes to that.
Sorry about the long reply -- it's something I feel strongly about, and have seen first hand how it doesn't always work out as it should if you rely on the school.