Unnecessary Comma?

Yes

Why?

Anonymous"Before class, Josh begged his friends for a pencil."Is the comma after the word "class" necessary?

No. It may help to use a comma after an introductory prepositional phrase, especially if it's very long, but it's not required, or even advised, by some teachers of writing.

At least that's the way I learned it, with a lot of red Xs on my comm

And I learned differently. If that phrase were at then end of the main clause, you would not use a comma, but when it comes before it, you do.

If I were editing, I'd put it in.

Only a single word, like "tomorrow" might get a pass from me.