Gloria QT'ing koi propperly normally takes eight weeks start to finish , in that time parasites and other problems should come to the fore enabling you to treat the koi which drops you right into the start of winter .
What size tank do you want to put this koi into and more importantly just what filtration do you have on the tank, how often would you plan to do water changes on the tank and maintain the filter if rhe koi goes into your classroom ?
If you choose to go down this route then it would be a good idea to add a smaller tank mate as koi tend to fare better that way being a shoaling fish
If your really worried about parasites then you could give the koi a short term 10 minute salt bath this should make the parasites if it has any abandon ship and die as they arent able to suurvive in salt water , interestingly Marine fish get fresh water dips to rid them of parasites see how to proceed with salt short term salt bath bellow :-
However the eight weeks QT period would still stand in our veiw, "why" to protect against any that survived the dip if they are embeded deep in the koi's mucus layer ...
Val and I tend to look at the appoaching winter as the wrong end of the year to be introducing koi prefaring instead to buy our own in the summer months so provided it gets all of the above them it chould do well .
Being an adult koi I take it its large , if so your going to need a tank that allows it to swim freely and have enough room to be able to turn around , so it would have to be quite big and the filter would have to be man enough to take the prodigious amounts of waste koi pass , the best would be a large external filter
However its down to you which way you go with this classroom or pond .
I hope this helps you with your choice of which way to go
For 22 years we kept our koi indoors to give you an idea of our tank size see photos
It was 6 x 2 x 2.5 and with filters cooler etc held 220 imperial gallons of water which works out at approximately 1 ton of water , so for the koi and a tank mate your going to need a tank 4 ft in length
Personally Gloria I'd tell your neighbour just what weve said and ask nicely if they could possibly keep this koi in their own pond until late spring summer of 2015 for you
Interestingly in a seperate thread Dr Dave says the same thing:-