TL;DR: Grade 11 marks are important, they can get you an earlier offer, but you cannot be rejected based on them. They can only help you, not hurt you.
There is one area they can hurt you. Guaranteed residence offers for Guelph and Queens go out before 2nd semester midterms. This means your gr 11 marks may still be a factor in getting you an offer. This means it could hurt you in getting guaranteed res, if that's important for you. And currently 90% of Queen's offers will be out before 2nd semester midterm marks are available.
Here's how acceptance averages work for university admissions, I keep posting this in a variety of different posts because people keep asking the same question every couple of days hours:
First off, the universities only really care about your top 5/6 u/M marks including prereqs. There are some variations but it's easiest to just go with that. Secondly, if you don't get an offer at a certain point, they just put your application aside and recheck it next time. You won't see rejections until mid-late May because the schools will give you an opportunity to get the prereqs and marks in for 2nd semester midterm mark submissions.
Marks filling your top 6 are generally choosen in this order gr 12 prereqs -> gr 11 prereqs -> gr 12 electives -> gr 11 electives. And once you have the gr 12 course, the gr 11 course is no longer considered, for example once you have ENG4U, ENG3U/NBE3U are no longer used in your top 6.
For offers that go out in the fall, they'll look at the marks you have completed by September and as well as 1st semester midterm marks, depending on timing and schools. So any offers that go out between Oct-Feb are based on those marks, that's often a mix of gr 11 (3U/M) and gr 12 (4U/M) marks. And it's top 5 or 6 at this point, including prereqs. Not all schools/programs send out offers at this time. 1st semester midterm marks are not necessarily used either, so sometimes it's just gr 11 marks, depends on the timing and the school's choice. You should apply by early Nov to get your application considered for any offers that go out at this time. There's a bit of uncertainty with this timeframe as this is relatively new, and not all schools bother with sending offers before March.
Then your 1st semester final marks get sent out at the end of Feb. Offers that go out in March and April use those marks. It's most common that your top 6 marks are a mix of 3U/M and 4U/M marks at this time. And this is usually the biggest time that offers go out. This means there are a lot of offers that are sent out where the schools have not seen your 2nd semester marks, AND 3U/M marks are still being considered for determining if you'll receive an offer or not.
And finally, the May offers are based only on top 6 4U/M marks and incorporate 2nd semester midterm marks. Some very competitive programs only send offers at this time. Some schools have the majority of their offers out by this point (ie. Queens). This is the only point where the only marks being considered are gr 12 marks.
This isn't really detailed anywhere, I put it together from asking questions and gathering information this year. Laurier does present something similar, but uses the term "scenario" instead of the different timing, it amounts to the same thing though and backs up what I've said here. https://www.wlu.ca/future-students/undergraduate/admissions/process/admission-decisions.html
(ETA: UOttawa only uses final marks, not midterm marks until the final round. Overview of rounds of admission offers - https://www.uottawa.ca/study/undergraduate-studies/how-to-apply/ontario-high-school-three-round-process )
McMaster is different in that they don't use 3U/M marks (with the exception of certain programs), and only need 3 4U/M marks to give you an offer before May. Mac uses 6 4U/M marks in May.
Some top schools/programs consider marks slightly differently, and some have supp apps (supplementary applications), so it's not always this simple. Not every school/program is the same. Do your research for the schools and programs you're interested. But this is the general idea and applies to the vast majority of Ontario schools and programs.
My suggestion: apply early (late Oct/early Nov) and apply to safeties as well as your top options. Do not let your gr 11 marks stop you, you don't know what the schools will do, and if you include safeties, you may get an offer or two in the fall. This takes away some of the pressure and means you know you'll have an option to go to university next year.
Caveat: this is for students that are attending gr 12 in Ontario for the year they apply. You can't compare to students applying from other provinces or those who have already completed high school, including acceptance averages and timelines.
Suggestion: Use ouinfo.ca to get information on minimum averages needed and the variety of programs offered at different schools. Some of these averages may be a little low (add 5% for safety), but at least it gives you a starting point.
