I'm only in grade 11, but I have a friend who is in university now who had a very close call with her conditional acceptance. She had to keep her grade to a 85 (I think), but couldn't and then had to beg one of her teachers to bump her up 3%. So what usually happens if you don't meet the conditional acceptence and none of your teachers are willing to change your mark? Do you have to do a victory lap..? Is it too late for you to look at the other offers your previously rejected?
oh also, by the way, for life sciences, it said that to apply you need a high eighty. What constitutes as a high 80? Does an 86 cut the mark (if so, what about a 85)? Thanks!

-Your in grade 11...chill. Your marks only matter for early acceptance right now and will have zero baring on any loss of conditional acceptance.

-Where most people have trouble with losing a conditional acceptance is slacking off Marjory once those conditional acceptances come out. Say the cutoff is 86%. If you got an early conditional acceptance that means your probably sitting at a 88+% right now. Now the conditional acceptance is likely not going to say "drop below 86 and lose your spot". They usually give you lots of room to play with. For example I believe eng was an 84 cutoff (4 years ago). My conditional acceptance in January was rated at about 75-79% which gave me over 10-15% to drop to get cut. With conditional acceptances Mac isnt trying to murder you, they are trying to give them an excuse to drop you if you pull like a 60% on two of the program's mandatory courses in second semester.

-Victory laps constitute another year of highschool. As such the ontario univeristy board will take your money and demand you to re-apply the next year. If you reject a university offer, after a certain date (or maybe immediately) you lose the ability to use it as an alternative. The spot they held in reserve will be sent out to another waiting student.

-High 80? to be safe 89%. Keep in mind averages change EACH YEAR based on the applicants average. However these averages do tend to climb each year a bit due to massive over-inflation of highschool grades. That so many people have said they got 99 or 100% in say an english is retarded unless they have professionally published a novel or other written literature. Even with math It should be extraordinarily rare to be a "perfect student".

-Keep your head up! I found that most people I talked to found grade 11 the hardest. G.12 isnt easy but marks seem to be easier to obtain.

Qwerty91 answered some of your questions but I`m going to elaborate a bit. For life sciences definitely aim for above an 88, the acceptance averages have gone up in the past few years. This year in particular the students who recieved early acceptance as well as conditional acceptance didn`t have a lot of wiggle room with the overall average they had to maintain. Like say you got early acceptance with an 89%, they wouldn`t allow you to drop lower than an 87.5% and if you did then your acceptance would be revoked.
If your teachers won`t bump you up then you either take a year off and hope that the average lowers or apply to less difficult schools or do a victory lap as you said.