Единтсвенное и множественное число сказуемого

I'm about to enlighten the rule that I've never rule cared about in Russian, i.e. when to use singular or plural forms under certain circumstances.

As far as I can remember, it depends whether I use numerals, groupings (мы с братом ...), or words such as сколько/столько/много etc.
Сколько студентов пришло на лекцию?
Большинство россиян живут по астрономическому времени. (although "живёт" would also be true; exception: nouns/adjectives after words such as большинство)

However, I'm still puzzled by the ambiguity when it comes to numerals in the following sentence:
У нас в классе сидит/сидят 30 студентов.

I know that if I were to use сколько/столько/много, it would be сидит.
According to gramota.ru, talking about that kinda stuff in the past might also lead to "сидит", unless I show emphasis by giving clues by words such as все/эти (...)
"Все 30 студентов сидят ..."

Which one is correct in this situation: "У нас в классе сидит/сидят 30 студентов."?
I feel that in one way, I'd put emphasis on the quantity, while the other would talk about the details, quality, although this sentence doesn't really reveal much about that, since it's all about "sitting".
So, there must be a single solution for practical purposes, since "sitting" isn't a big deal.

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.