Same question. I had assumed that та was used sometimes for phonetic reasons, but cannot imagine that being the case with the и/а distinction, which I had thought was arbitrary in this case (where the preceding word does not end with a vowel, let alone the same vowel, and the next word does not begin with one, let alone the same).

"І", "й" and "та" are used when you enlist items or actions. Like "There is an apple and an onion on the table" - "На столі яблуко і цибуля". I think one can use "i" here, but it would sound like two pointless phrases. "Students are there. I am here. No connection, just informing you." If, however, we use "a" the second statement is not independent from the first one, it says "Ye, those student are there, but I am rather here". The second sentence contrasts with the first one.

Ось is closer/nearer than тут. And ось is more defined than тут.

Ось ця річ -у мене в руках. The thing is here - in my hands. (and show this thing or your hands). Ось документ - на столі. Document is here - on the table. (and indicate of your finger on the document)

Ця річ тут у кімнаті. The thing is here in the room. (I don't know exactly in which plase of the room). Цей документ тут на столі. The document is here on the table. (I don't know where exactly on the table. Maybe under the books, maybe between papers or somewhere else).

With a verb of motion - yes. If it was "Students go there" that would require сюди.

Учні (students) is acceptable