As always the use of an apostrophe is determined by the meaning you want to give the word.
Pupils: more than one pupil e.g. There are 30 pupils in this class. (There is more than one)
Pupil's: owned by a pupil e.g. Have you seen this pupil's book? (The book that belongs to the pupil)
Pupils': owned by more than one pupil e.g. I want to talk to you about the pupils' behaviour. (the behaviour 'belonging to' more than one pupil)
NB Pupil can mean two things. A pupil can be a student at a school, which is how the word is used in the above examples.'Pupil' is also the name of the transparent portion of the eye (the black dot in the middle). They are treated the same grammatically, but the example sentences would be different.
There should be an apostrophe in that phrase. If you're referring to one pupil, it's "pupil's classroom." If you're referring to a group, it's "pupils' classroom."
If you're talking about something that belongs to the teacher, then you need the apostrophe. Teacher's desk, teacher's computer, etc. If you're talking about teacher as a plural noun, then you don't need the apostrophe.
The possessive form is the pupils' assignment.
classmates,students,pupils
This depends on the rest of the sentence E.g. Your instructors are happy with your progress or Your instructor's watch has gone missing, have you seen it? or Your instructors' meeting was a waste of time as none of them brought their notes
There should be an apostrophe in that phrase. If you're referring to one pupil, it's "pupil's classroom." If you're referring to a group, it's "pupils' classroom."
Pupils'. When you have a plural possessive, then the apostrophe goes after the s. If a pupil owns something, that is the pupil's stuff. If pupils collectively own something, that is the pupils' stuff.
I am does not need an apostrophe. It is only when you make it into a contraction by dropping the a that you need the apostrophe. It becomes I'm in that situation.
No. The Cannons doesn't need an apostrophe.
No, Christmas Eve does not need an apostrophe.
No, because plurals do not need an apostrophe.
As a plural, for more than one dad; no, does not need an apostrophe. As a possessive, as in the item belonging to dad, then yes, it does need an apostrophe.
No. Harringtons doesn't need an apostrophe.
The plural "Thursdays" doesn't need an apostrophe.
Lost wages doesn't need an apostrophe.
No, "Doc Adams" does not need an apostrophe unless it is possessive, such as "Doc Adams's bag."
do you need an apostrophe after the s in Koreans