Glassjaw is a metaphor, it is not a real thing
When practicing a jab you should not hit you in the face you should block it or move out of the way.
It should also not hit you in the jaw/chin.

If you are unfit or exhausted you might get headaches or may be light headed.

1st Stop getting hit in the head.
2nd it should not hurt for ten seconds, you are miscalculating the amount of time it hurts.
3rd Evaluate your training getting punched in the head is not training.
4th Try exercising, if you feel light headed you will know it is not the jabs causing you the pain.
5th Drink water before you do any sport, drink more water in general.
6th If a punch is coming at your head, move out of the way, block or deflect the punch.
7th Stop telling people to punch you in the head.
8th I normally would not mention this but are you using gloves.
9th Ask them to punch slower, and gradually get faster as you feel more comfortable.
10th Have fun.