Firstly, the more students there are, the less time the teacher can devote to them individually. When there's a comprehension problem, it's really in the interaction that things can be sorted out: understanding why the person doesn't understand, explaining from different angles, etc.
And all the time spent with one person leaves the rest of the group on its own, which can very quickly dissipate all the group's attention, with a few whispered personal discussions turning into hubbub.
What's more, the larger the group, the easier and more tempting it is for weak and shy students to hide in the crowd. In a small group, the teacher should regularly check that no one is left behind. With 100 people, this is strictly impossible.
Finally, I don't believe in the argument that the more people you have, the more friends and studying partners you can find. Socially, we get together in small groups (between 2 and 6, roughly speaking) no matter what's going on. I think that beyond this limit, there's a diminishing return (unconsciously): a new person brings less than reinforcing the group's cohesion. If the group gets any bigger, it's going to reshape itself by affinity into smaller, tighter-knit groups.
Rating: I don't believe in rating system. If I say 3/10, can you change 20% of your idea to reach 5/10? What's 20% of an idea ? Would 5/10 be good enough? Can you change 70% of an idea to reach perfection? In this case, why don't we live in a perfect world?
Also, avoid direct eye contact, and never ever stare into their eyes. This is the equivalent of an oath of blood and despair.
That's something people who don't own a cat don't understand: the less you look at them, the more friendly they think you are (that's why cats love allergic persons)