Iīm not sure what to do, normally If I would be advising someone I would just say "get a small slice of each and work out extra hard and eat only healthy low-calorie food the rest of the day and just count it towards your daily calories, but the fact is I donīt want the pizza or the cake, I would rather have a satifying 400 calorie delicious healthy meal that I make myself than one slice of greasy pizza for the same amount of calories. I just donīt think itīs worth the calories! so I donīt know what to do
I canīt just go there and sit down with all the students and not eat, I just donīt know how I would explain that since noone knows about my new healthy lifestyle and my friends love to eat and would think I was being ridiculous! sorry this was a long post Iīm just really confused!
All advice will be greatly appreciated!

Just be glad you had advanced warning 

Keep up the great work!
And no one really questions them as long as they don't make a big deal out of what they are doing.