I do tutoring in a class of about four people for 4u and for the past few years (9, 10, 11), I had around 10 people in my class. I found it extremely helpful because they provided units of work that was way in front of my school and the teachers explained things very well. We would cover a multiple of things per topic once every week and the questions they chose for examples and homework were really nice, ranging from easy to challenging. I guess if you have the motive to look at the topics that are ahead of your school and have the ability to understand it from reading the text without any explanation from a person, I guess this wouldn't work out too well.
Overall, it has definitely boosted my marks as I have had time to understand the work and master it by the time our school reaches it. :]