They all look grammatically correct to me. Well done! As I imagine you know, all these sentences mean slightly different things.

I think the correct form of what you are trying to say would be: "Math seems to be pretty difficult for most other students." (Note that depending on region, one might say "maths" instead of "math." For example, in South Africa I would say "maths," but in Canada I say "math") I believe when you are constructing a sentence involving "difficult," you use "for" if the following term is a noun and "to" if it is a verb. For example, "math is difficult for other students" and "it is difficult to do math." Personally, I would probably word the sentence, "maths seems to be quite difficult for most other students." I personally find it a little more natural to use "quite," but that might just be me ("pretty" would also be much less formal).

Typically "freaking" would be used to express emphasis, frustration and/or annoyance. It's informal and I wouldn't recommend using it around people you respect.

I find that the easiest way to learn some colloquial terminology within your target language (for you, English) is to start speaking it with people (and reading) and picking them up as you do so. One example off the top of my head is the word "ain't" which is an informal contraction for just about everything (is not, are not, am not): "Say it ain't so," "I ain't telling," "They ain't seen nothing yet," etc. I don't particularly recommend using it actively, though, as it is very informal and sometimes associated with being uneducated. However, it's good to know what it is in case someone else uses it.

You might say, "I (still) have a lot of work left." (the "left," i.e. leftover, implies that your work is not yet finished).