I was told to use 7 USG/H (26.5l) for the ones I used to fly. The book figure was nearer 5.5 (21) for average conditions but they were not new aircraft and could not be expected to perform to the book.
If your instructor said 24 litres, and it was the same type of aircraft with the same book figures, then he was being a little less cautions than my instructor; there's no reason to suppose that he wasn't correct for your particular aircraft.
Anyone calculating on 16 litres must have been flying a completely different variety of 152.
Which is not to say he was wrong. You start by looking in the POH for your individual aircraft and use a higher figure if so advised or if that's what you find it burns. There are probably other reasons for adding fudge factors but I don't know what allowances to calculate as I've always been told the fudge factors by instructors. If you friend has done this, starting with his book for his individual aircraft, and come up with 16 then he's probably right, but in my very limited experience I've never seen a 152 POH that suggests it burns that little.
Others will no doubt be along in a moment with better information.
One thing that's for sure is that it won't burn less than it says in the book.