Eh, they aren't savages and there are plenty of blood in the chalices around.
Besides, you've saw what Bram did to George when he tried to sneak on him. If any of them tried, they would regret it instantly.
That depends on who would discover him. If it was an adult with some good sense, they wouldn't want to anger Bram's parents and would just give him a stern talking to before expelling him from the party. If it was an adult without any good sense, they would kidnap Bram to try to get some revenge or some leverage against the vampire hunters (which would backfire espectacularly, but Bram could get hurt in this).
Now, being discovered by the children would be the worst because they could bully him, could pick up fights (like George tried) and this wouldn't be even punished that harsh, because "it's just children".