DD's school isn't too bad with uniform (they're strict but it's pretty simple) but I don't get this rule she's just told me.
After Easter they're allowed to take off their blazers during the day, but if they take the blazer off they have to take the jumper off as well (the jumper isn't mandatory). So they're not allowed to just be in their jumper, ever. The jumper has the school logo on it, as does the tie.
And they have to wear their blazers in assembly, whatever the weather.