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

They're young. Things like free throws and throwing a hitch in football are a repetition thing. Quality mechanics will shorten the number of repetitions needed for mastery but, even so, it is asking alot for a 12-14 year old to have mastery of a repetition skill. Athleticism can mark a lack of repetitions but for most kids that isn't the case. Also, you can expect a regression of the skill in a game setting. At least until they figure out that the skill is used the same way in a game as it is in practice...that is an experience issue.

What I am saying is this: reps, technique, and time will eventually smooth out the problem. Be patient.