The issue of peer pressure should be taken head on. I prefer not to tell white lies about last minute work. Answer honestly by starting with a simple statement "honestly, I really don't like sharing my work until after it is graded. I guess I am just very private about this sort of thing. Sorry, hope it doesn't offend you. I hope you can understand, I'm just like this."
If your classmates are reasonable adults, they will respect your preference. If they proceed to publicly ostracize you, give a retort along the lines of "In this day and age, we are supposed to be tolerant of each other. Your attitude is childish and distasteful."
Please stay away from being vocally judgemental towards these grad students. Instead, be a role model.
Moreover, in that in most graduate programs, there is a camaraderie that build while you are taking classes and before qualifying exams. Make clear that you are willing to work together with others in solving difficult problems. Alternatively, if you are way ahead in this particular subject, hold informal review sessions to help other grad students solve their learning issues. These two ideas will make clear you are a helpful person, and if they are just lazy, you won't need to do anything.
Eventually, there might be a course that you find difficult and need help. Then, you can lean on your peers. Alternatively, when you start performing research, there will also be tasks (paper writing? coding? math? oral presentation preparation? moving apartments?) that you will need help with. You will need your peers, so help those who help themselves (and maybe you too). As an added bonus, nothing helps learning like teaching at a black board before your peers.
As for grades, IMHO you can forget about those. You're already in grad school.