Yes, that sentence is grammatically correct. It effectively conveys the message that all the meetings you had planned for today have been canceled.
The sentence given is grammatically correct. The verb in in its imperative mood and therefore does not need an explicit subject; the subject "you" is presumed.
Yes, that sentence is grammatically correct.
Yes, the sentence "Today is hotter than Tuesday" is grammatically correct. It compares the temperature of today with that of Tuesday, indicating that today is warmer.
No, it is not.
It is grammatically correct to say , "I am in school today." This is because you are in the building, not at the building.
If it's just those four words, then it should be "Who is here today?" If it's part of a longer statement, then either could be correct, depending on if it's plural or singular. "The boys who are here today" "The boy who is here today."
No, the sentence is incomplete. It needs additional information to be grammatically correct, such as what happened during lunchtime or where it took place. For example, "Today during lunchtime, I met a friend at a café."
No, the question "which day is it today?" is grammatically correct and is asking for the current day of the week.
Not exactly. It should read " .... help you with today?"
The word today is a possessive in the noun phrase "today's meeting" (meeting happening today). The word today is an adverb in the verb phrase "meeting today" (occurring today). The latter is unusual but not unseen in sentences such as "The President will be today meeting with his Cabinet" instead of "The President will be meeting with his Cabinet today" especially if there is a longer object such as "meeting the most vocal critics of his plan to end the ongoing crisis" where the "today" could be too far from the verb modified. More typically, the "today" is used to start the sentence.
The correct phrase is "today's meeting." Using the possessive form "today's" indicates that the meeting is happening on the specific day that is being referred to.