re: Is it acceptable for me to punish junior high(7-9 grade) kids for a loss?

Do you think they wish they would have made more free throws? I think punishment is better left to instances where people intentionally do something wrong (i.e. missing practice, being disruptive, etc.). Your job is to figure out how to make them better free throw shooters, and not to simply take out your frustrations on them.

Maybe you need to allocate more practice time to free throws. If they do well in practice, but not in games, maybe you should include something in practice to teach them to tune out distractions while shooting.