I always put my gown on at break, but when you have to go straight into another pose I don't bother. Sometimes the tutor will give you your gown , once finished a session, it just depends really.
As far as changing facilities are concerned, most are quite rudimentary - store cupboards/rooms can be tolerable if there is enough space but toilet cubicles are not really acceptable. But sometimes one comes across decent arrangements, such as the Princes Drawing Studios near Shoreditch in London where there is a shower, toilet, and reasonable space.
As regards gowns, etc.it depends on the kind of session. More often then not, I am naked during breaks if they are in the studio; I get requests for this if artists ask me to. And sometimes my body is oiled if it is a photographic session, in which case a gown is not really sensible.
There are also sessions for LGBT groups where I am asked to use a G string during the break rather than a gown, and others where I am girt in a wrap-round cloth with legs, arms and shoulders fully bare.