Yes, it is possible to replace "only" with "just" to mean "only in this case," as in your example sentence: The class is only on Thursday. The sentence could very well be The class is just on Thursday.
One meaning of "just" is "Merely; only: just a scratch*"
The same dictionary defines only in these ways:
1. Without anyone or anything else; alone: room for only one passenger.
a. At the very least: If you would only come home. The story was only too true.
b. And nothing else or more: I only work here.
3. Exclusively; solely: facts known only to us.
a. In the last analysis or final outcome: actions that will only make things worse..
b. With the final result; nevertheless: received a raise only to be laid off..
a. As recently as: called me only last month..
b. In the immediate past: only just saw them. .
In all of the examples above, "just" can be substituted for "only." As stated in A Communicative Grammar of English**: "Other words with a meaning similar to only are just, merely, simply.." Longman English Grammar*** states: "Adverbs such as even, just, merely, only, really and simply can precede the word [or phrase] they qualify to focus attention on it."
The erudite nitpicker is wrong.
*The American Heritage Dictionary of the English Language, Fourth Edition Houghton Mifflin Company. 2003
**A Communicative Grammar of the English Language, by Geoffrey Leech and Jan Svartvik. Longman Group Limited. 1975
***Longman English Grammar, by L.G. Alexander. Longman. 1988