Search messages in your channel:
Select a channel you wish to search up a message in.
Towards the top right corner of you should see a row of icons.
Select the 🔍 icon to search for a message.
Type in message content you wish to search into the "Search Message" bar.
Search for messages from a specific student:
Type "From:student name" in that exact format into your channel's search bar (no space in between).
A list of all of the messages from that user should appear in a list beneath the search bar
For a discussion, search works the same way. Just go into the discussion page itself, and follow the same instructions as above.