On the other hand, some students might be curious. They wonder who could be calling Percy during class. Maybe they'll start whispering among themselves, making speculations about whether it's a girl he likes or something more mysterious.
Some students might laugh. It's a common reaction when something unexpected and a bit inappropriate like a phone ringing in class happens.
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.
Some characters might find it cute. For instance, Grover could just smile to himself seeing how close his friends are. He might even be a bit envious of their relationship.
In fanfiction, mortals might react with shock. They may be curious about how Percy got those scars. Some might be sympathetic, while others could be fearful, thinking Percy has been through some really dangerous situations.
Some students might be shocked. For example, Ron would probably be open - mouthed in disbelief. Hermione might be initially scandalized but then try to analyze the situation rationally. The Slytherins could have a mix of reactions. Some might be disgusted as they don't expect Blaise to be involved with Harry. Others might see it as an opportunity to tease Blaise or get some leverage over Harry.
Annabeth would probably be very worried and take charge. She'd start researching immediately, looking through books in the camp library to find a cure. Grover would be constantly by Percy's side, using his nature - related powers to try and make Percy more comfortable, like bringing in fresh air and plants to help him feel better.
They might be a bit in awe at first. She has a certain presence about her that comes from her being a demigod. Maybe they're curious about her and ask Percy a lot of questions about her later.
His friends would be very worried. Annabeth would immediately start looking for solutions. She'd be using her strategic mind to figure out what kind of poison it is and how to get an antidote. Grover would be freaking out a bit but still try to be useful, like using his sense of smell to detect any strange scents around Percy that could give a clue about the poison.
Some characters would be shocked. Imagine Legolas seeing a fanfiction where he has some really strange adventures that are completely different from what he actually experienced. He'd be taken aback at first.
Some students might be shocked. The Gryffindors, especially Hermione's friends, would be very surprised as Draco has been their rival for so long. They might initially be against it, but later come to accept it if they see how happy Hermione is.
The students would probably be very excited. Percy has a lot of charisma and his tales of adventure would be really interesting. Some students might be a bit intimidated at first, but once they see his friendly side, they'd warm up to him quickly.