Ive been in this situation before in a 152 that lost about 70% of its power on takeoff about 3 months ago. Lost power on the base leg about 4 seconds before i was going to start my downwind turn. Did my flow to make sure everything was working. (fuel selector, carb heat, mags, primer.) Didnt let my instructor fly since i was a cfi applicant. landed on a different runway since there was intersecting runways. Came out fine and plane went to maintenance and had spark plug wire problems.

Just remember if your a cfi to teach students that if some one is calling an engine failure you teach them that they should leave the airport and go some where else besides there. Some student on a solo was calling final when i was pretty much gliding on final on the intersecing runway. Wanted to give the student a few words when i was done, but let his instructor know instead.