Players are able to add emoji reactions that can be put in a order and imply something that is inappropriate
Default Discord emojis have letters, you can spell a lot of inappropriate things yet I don't see anyone doing it.
Honestly, if staff has it's hands full with moderating both Minecraft server and Discord I suggest:
Turn on Slow mode in every single channel
Disable reactions to messages
#art - only allow images, no text
#general - only talk about server specifically
#random - delete it, who wants to moderate the memes that are posted there, am I right ?
Allow chatting only when staff/moderator is present
Allow messages to be posted in certain format in order to be professional + Proper salutation, body paragraphs with the mention why the player is writing a message and a signature at the end
Stop looking into a crystal ball trying to know the future. Player emotes have not yet been enabled, yet you all know that every single troll on the server is waiting specifically on that and how that everyone will forget their manners once such thing is allowed.
Instead of trying to find the reasons as to why YOU (as in mostly staff who disagrees) don't want it, why didn't you propose to just try it for a week or two, to see if it'll be a fiasco or not.
Edit:
Community involvement is something I am 100% behind and definitely would like to see more of, but this just requires too much work for too little payoff.
Right click > Edit channel > Permissions > Student > "Add Reactions: ✓" "Use external emojis: ✓"
6 clicks isn't that much work
Right click > Reactions > Click X on the reaction you don't want
>3 clicks isn't that much work