Videl runs off to fight crime. Gohan runs to the bathroom, preforms the multiform technique, and 1/2 of his power gets to go and back her up while the other 1/2 goes back to class. As Saiyaman he probably never used more than a hundredth of his power, so a 50% handicap wouldn’t have hurt anything. Heck, he could have maintained his golden fighter identity and just made sure to let Videl (and anyone else) see both him and the gold fighter side by side to secure an iron clad alibi.
I’m not sure if he ever used the technique himself, but I know he saw his Dad use it during the Cell games and I’m pretty sure he knew about it before that from Krillin. Instead of flying to Bulls for the costume after his first day, he could have flown to Kane house and asked Krillin to teach him the move
I know that the real answer is ‘because it would have broken the plot’. I’m looking for a believable in universe reason. I don’t buy that he wouldn’t have thought about it, he’s a genius. Why didn’t Gohan use the multiform technique during the Saiyaman saga?