A native speaker can use any of those variants. The problem is that many people treat numerals as neuter nouns ("тридцать сидело", "восемь стояло" etc.) while others don't like it because it has too colloquial color. The word "студентов" doesn't look like a subject because it is in Genitive and people should make an effort to agree a predicate with it. If we replace numeral with a noun, the predicate will naturally agree to that noun: "Дюжина студентов сидела..."

So now the situation in language is ambiguous and when it will be stabilized the formal grammar will accept the choice of the people.