Well, he might get in trouble with the teacher. Teachers usually don't like phones ringing during class. The teacher could confiscate his phone or give him a detention.
I think Percy's classmates could be quite affected too. Their concentration gets broken, and if they were in the middle of an important lesson or test, it could really annoy them. Some might also be worried that the teacher will be in a bad mood for the rest of the class because of Percy's phone, which could affect how the teacher interacts with them for the remainder of the period.
One idea could be to have Percy Jackson end up in Middle - earth. He could use his water - related powers to help in battles against Sauron's forces, like using water to put out dragon fire. His demigod skills would be a new element in the LOTR universe.
When creating the plot, find a good balance between the two worlds. Don't make it too Percy - Jackson - centric or too LOTR - centric. Maybe start with a problem that can only be solved by the combined efforts of characters from both series, like a new threat that needs demigod and LOTR hero powers to defeat.
It could be Chiron in disguise. Since he has a vast knowledge of Greek mythology, he would be a great teacher. He could also use his centaur form in some secret or magical way during the class to teach important lessons that only Percy and Annabeth would notice.
Well, in many fanfictions, Percy Jackson's class could get involved in a quest at the camp. Maybe they have to retrieve a lost artifact or solve a mystery that's been plaguing the camp. They could also form new friendships and rivalries with the other campers. There might be some intense training sessions where they learn to use their newfound powers. And who knows, they could even encounter some long - lost gods or goddesses who have a special task for them.