Question: > 
> It's 4 am \_\_ the clock.  
> 
>  It's 4 am \_\_ my watch. 
> 
> 
> 

Which preposition is to be used?

Answer: 
Both are grammatically correct, but it depends on the situation which one would be appropriate. 

Generally speaking, we use the plural with everything except one, and **no** counts as not one.

> 
> **No students** were present.
> 
> 
> 

There are, however, two exceptions. The first is if there could only possibly be one or zero. For example, when talking about the moon, we use the singular:

> 
> There was **no moon** last night.
> 
> 
> 

So, if there were a university committee which normally had one student representative, it would be OK to say

> 
> There was **no student [representative]** present
> 
> 
> 

The second is when we want to be emphatic: we can use the singular to emphasize that there was **not one single...**. So, if the teacher was disappointed to find that no student turned up, he or she could say

> 
> Not one single student was present  
> 
> **No student** was present
> 
> 
> 

Here is an idiomatic expression that is a good example of the use of the singular for emphasis:

> 
> to [leave **no stone** unturned](http://www.learnersdictionary.com/qa/idiom-to-leave-no-stone-unturned) 
> 
> 
> 

Here is an example using almost the same phrasing as the question:

> 
> While **no student** was allowed to leave the schoolhouse, representatives of the press and other news media, on invitation of the school authorities, were permitted to enter the classrooms to observe the proceedings. [We the Students: Supreme Court Cases for and About Students - *James B Raskin, 2008*](https://books.google.co.id/books?id=B0N1AwAAQBAJ&pg=PA149&dq=%22no%20student%20was%20allowed%22&hl=en&sa=X&ved=0ahUKEwi69uWmy7HbAhVJfCsKHWM8CdwQ6AEIRjAF#v=onepage&q=%22no%20student%20was%20allowed%22&f=false)
> 
> 
>
Both are correct in US and British English. If one student was expected, or at least one student was required to be present, you might choose to use 'no student was present', and if more than one student was expected, you could use 'no students were present'. However, this is not a strict rule, and you can use either.