I've seen it recently with a friend's daughter's pup :-(
During the week, he's looked after by friend as daughter is working - leaves early, gets home late. The daughter has his care during the weekends, half term etc.
Friend is very calm, ordered, he has a routine and feeds/sleeps regularly. The daughter treats him like a toy/entertainment system, high excitement, no routine, wakes him when he's asleep, feeds him non stop and treats with whole sausages

What a difference!
After a weekend, on a monday, the poor pup is so wound up he's biting non stop, running around like a lunatic etc. By monday lunchtime the poor mite has a massive 'crash' and sleeps for about 4 hours straight. He'd probably sleep'longer, but the boyf arrives and it's high excitement again :-(
We've just had easter hols and I dread to think what shape the pup is in now. Daren't phone as he'll be sleeping and in recovery :-(
Will be calling in tomorrow to see how he's getting on.
I have told friend to carry on regardless of what D does. She want to leash train, have him in a crate in the car etc. DD won't :-( He will learn that her house is calm and will be lovely for her, but be a hooligan for D
So much for teachers being the responsible adults that they are supposed to be :-(