We seat the freshmen, sophomores and juniors like this:

ffssssssjjj

That's 2!∙6!∙3! ways

For each of those ways, there are two cases:

Case 1: the two seniors, say Ann and Bill, sit side by side:

_f_f_s_s_s_s_s_s_j_j_j_

There are 12 places they can sit together.
There are 2 ways they can sit, Ann on Bill's left and Ann on Bill's right.
That's 12∙2 ways for case 1.

Case 2: the two seniors, Ann and Bill, do not sit side by side:

There are 12 places Ann can sit. For each of the 12 places Ann can sit,
there remains 11 places Bill can sit.
That's 12∙11 ways for case 2.


Answer:  2!∙6!∙3!(12∙2 + 12∙11) = 1347840 ways.

Edwin