I like the one where the FF go off for a long weekend, find an old house, decide the ancient sink and crockery is usable with a wipe and wash from a rusty pipe, then make the corner cosy with branches and leaves They don't have time to bring spare clothes and come in their school blazers. They row on a lake, follow a map, find hidden jewellery in a boathouse or somewhere, cleverly hide the jewellery, from crooks ( obligingly stupid ones) - ingenious plotting - and then....dust off the blazers and back to school.
That's Five on a Hike Together, which is also a firm favourite of mine. I think the brevity of it all is that the school the girls attend has a halfterm weekend, and the boys get some extra time off because someone has won a scholarship, so the whole thing takes place over a long weekend when their leave coincides.
And I think that pupils quite often wore school blazers out of school in EB's era -- fewer clothes, clothes rationing, not really much of a concept of 'leisure wear' or specific walking/hiking gear, and blazers were hard-wearing and useful? There are definitely references in some of the other FF books set during the summer holidays to the children wearing blazers when it's chilly.
(I remember noticing that Kathie Ferrars in the Chalet Schools books is described as still wearing her old school blazer, despite the fact that she's been left school for long enough to attend teacher training college and get a job.)
I love that one. It has a thick-necked ex-con villain called 'Dirty Dick' (in case we were in any doubt as to his villainy!) and a female bad'un who dyes her hair and chainsmokes and is 'hard-faced', so we know she's the type to hang out with escaped prisoners. 
And I love the bit where Dick and Anne are trying to make for a farm that offers accommodation and meals called Blue Pond, get lost, and continue to think that the godforsaken farm where they end up overnight must be the rightplace, despite the fact that it's staffed by a terrified old woman who hides Anne in an attic and Dick has to sleep in the barn!