Why is he doing this?

Why does Bentley, our 4-month old Cavalier, try to mount the other dogs in obedience school? As soon as we get in the room he's all over them. He goes absolutely nuts. The others don't do that, and he's the smallest of the five puppies in class. There's a toy poodle (who looks bigger than I thought a toy would be), a bassett, a border collie/poodle mix & another one I don't know the breed. It's rather embarassing, as if my child was acting up in the grocery store!

Jasper was a Humper and still is. He was so embarrassing at dog obedience he would get on my leg then when I tried to get him off he was on my arm! Lol.
They even hump each other when they are playing and it doesn't matter which end they hump! and Holly's a girl. I wouldn't worry too much about it. Karlin can advise you on this.

Humping is 1 of 2 things, or a combination of both. Dominance, and/or raging hormones!

There is plenty of success with simply just training your pup that this is an unacceptable behavior. Dogs in the show ring that are still in-tact must learn to control their hormones! A 4 month old puppy may just be starting to get some hormones...but that does seem rather young. Female dogs will hump also, which so many people find surprising as this is a behavior usually linked to male dogs.

A swift NO, removal, and time out (no more than 10 seconds) then big reward after for behaving is a good start for training

It's important to check it - gently but firmly - at this early stage because even if you have Bentley neutered at a later date, if humping is already a learned behaviour (and from the dog's point of view a pleasant habit!) neutering probably won't make any difference. With Bentley it may be an attempt to assert himself among bigger puppies - or a prococious start to the teens!

Kate, Oliver and Aled