In my experience, international students are very active in grad student unions. Also, good luck bringing in scabs during finals week. Admission runs on a sch

In my experience, international students are very active in grad student unions. Also, good luck bringing in scabs during finals week. Admission runs on a schedule, visas add delays, and even if they show up in time, they'll soon be informed that it's a closed shop and scabbing has financial penalties. Oh, and there's typically a high degree of solidarity between profs and their grad students -- you don't just get admitted by a university, you get admitted by a committee of professors in the department you apply to.