It seems a bit unfair to steal a meal when the girl can't do anything to stop him. [Or at least it's unfair that it's not you stealing the meal.] Granted, one hardly wants an equal fight, but somehow this case seems to cross a line. You'd like to help her out, but how? Most any attempt to stop the guy will cause a fuss that just earns all of you a trip into Detention. Clearly not a good idea. So how to help and actually help?

After a moment's thinking, you get an idea. You write a quick note to Lizzy, asking for her panties. Lizzy is quick to comply, giving you just enough time to write a 2nd note.
Now you grab the guy's shorts to get his attention. Then you push the note in front of him "Spit her out or you will be wearing these!" and show him Lizzy's panties, which are girly even by female standards. Another tug at his shorts makes him realize he can't stop you, save by methods that will send both of you to the snake cage. And once you got those on him, you could easily cause a fuss that would send both of you up front, with him wearing these challenges to his manhood for all to see.

He sees things as you expected, and unmouths the lass. Both of you return to your seats, with him glaring at you for the rest of the class, and then storming off when class is dismissed finally.

However the lass, a lovely brunette who is rather overdressed by school standards [tho that likely would still horrify her mother] approaches as you exit the room. "I'm Julie and I want to thank you for rescuing me back there. You really deserve a reward, and the traditional one is for the rescued girl to let you eat her.
"I do want you to promise to take my clothes and books to the Nurse's office so I can pick them up after you have digested me. And I do embarrass easily from showing too much skin, so try not to take off my clothes until you have as much of me down as you can. But otherwise, feel free to have an early lunch, and my thanks."

This is a pleasant result, but maybe it can be made better. "Thanks. However, this is my girlfriend Lizzy and she needs practice at vore. So a volunteer like you would be really help. Since she helped too, can she swallow you?"