If its a "competitive" game i.e. for a formal league or a cup of some sort, then you ref it as a normal game that you'd been appointed to.
If its some form of inter schools friendly, then i'd take a slightly different approach and speak to the teacher beforehand with your proposed approach (never ask them how they want you to ref it, always be assumptive) and would do it like this.
"Just want to confirm its x minutes each way and rolling subs. As its a friendly i'm planning to manage misconduct in the following way:
If a player is getting close to a yellow through persistence or ongoing dissent, i'll give you a chance to get hold of him before I take action. If its a yellow card tackle, then he's getting a yellow, likewise if theres VC or OFFINABUS i'm dealing with it, is that ok?
The only thing i'd be a bit more lenient on in a friendly would be would be a DOGSO tackle outside the area where I felt the player has made a genuine effort to play the ball, if however he's hacked the player down deliberately, then thats a different story.
With Under 18's and below football, I also tend to gauge whether the Captain is actually able to take any responsibility and help control the players, or whether the normal on the pitch communication with the captain would be better directed to the coach on the sideline.
I did an U16's game where after I'd cautioned one of the players, he was still diving into tackles, as I jogged past the coach, I said "if he carries on he's likely to force me into making a decision" to which the coaches response was "yep cheers Ref, its a lesson i'm willing to let him learn the hard way, so do what you need to do"