Ava Alexandra Hughes is seven years, four months and 15 days old. She's looking forward to her first day of year three at Crabtree School for Girls.
But there is a serious problem: Crabtree School is just too good. It's such a good place that the a class from last year refuse to leave.
Now those pesky 11-year-olds are clogging up the whole school, and Ava is going to be stuck in year two forever and ever. Ava and her friends must come up with a plan to get the year sixes out for good.