Another one who is thoroughly confused by all this. Every class will have a wide range of abilities. Children who are strong in maths may struggle with literacy, or music, or art.
From what I've seen after having three children progress through Primary school is that most teachers are very well equipped to deal with different levels. There is very little "whole class teaching" for English and Maths - the children work in groups and have targets according to their abilities. So on a Monday morning when they're doing their "what I did at the weekend" diary in P2 or P3, the target for Jane who finds it difficult might be to write at least two sentences and use capital letters properly. For John, who is an able pupil, his target might be to write three paragraphs and use lots of adjectives.
If they are doing topic work, art, music or anything else the teacher should be pushing the able ones to research more, add more details, prepare a powerpoint presentation, share their learning with the class and supporting the less able ones to get the basics done. This is what I see happening EVERY DAY in my child's school.
My eldest started P1 reading fluently - we didn't teach him, he's just weird like that. So he read in a group by himself, with a classroom assistant. But he wasn't on his own all the time - just that 10 minutes a day while he read aloud.
There is no "p2 work" and "p3 work". If you feel that your child is not being stretched then that is an issue with that particular school/teacher not the system in general.