If you want to have the option of taking a community college course or two this summer, start looking into the process soon – I would recommend no later than March. In California, taking a CC course during the summer as a high school student requires jumping through a few hoops that can take some time. If you have a HS guidance counselor who is familiar with the process, or can connect up with a CC admissions officer familiar with the process for high school students of your age, good. Otherwise, be prepared to have to drive the process yourself.
In case you don’t have an experienced or helpful counselor…
Start looking at the admissions and registration information on the website of whatever CC(s) you’re considering; if you dig around, they will have information (some of it possibly self-contradictory) about their requirements/restrictions for admitting and registering high school students.
You will likely need a form signed by parents and by high school. You MAY be required to attend some sort of orientation session at the CC; in theory, high school students who will be continuing in high school (not about to graduate) probably aren’t actually required to attend orientation, but depending on the CC (AND on who you ask) you may get told you are required to attend an orientation session. You will likely need to take the CC’s math and English placement test just in order to be allowed to register. You will need to find a time for the placement test that works with your schedule. However, the math portion of the CC placement test may only go up through placement into 1st semester Calculus. That means that if you want to take a course for which the CC considers two semesters of Calculus to be a prerequisite (e.g., 3rd semester Calculus, Intro to Linear Algebra, or Intro to MATLAB), you may need to go through some exception process with the CC to get your HS AP Calc AB credit accepted as a prerequisite and/or you might find yourself needing to work with the math department at the CC (maybe take a test from them) to show that your Calculus skills are good enough to go into a higher course; this is a situation where if your high school already has a good relationship with a particular CC, this may all be a non-issue or easy (maybe they’ll just accept a good grade from your HS AP Calc AB!), but if you’re trying to take courses at some CC that doesn’t get many HS students you may find it taking more work and time as you go in circles trying to get someone at the CC to approve you. Either way, you should start the process soon enough that you aren’t prevented/delayed in registering from CC courses due to (apparent) lack of proper prerequisite courses.
In case you do end up needing to show via a test that your AP Calc AB class taught you enough for whatever next course you want to take, you’ll need to consider how soon you’d want to take the equivalent of an end-of-year Calc test – and do you want/need to do some studying in advance, or (since you won’t yet have finished the Calc class, in case you don’t do quite as well as you’d like) will you still have enough time to go through the CC’s ask-for-a-placement-exemption process? If you can show you are doing well in your HS Calc class, and your HS is well-regarded, then the CC math department people are likely to be helpful regarding an exemption as they’ll understand if you aren’t quite finished learning Calc since your HS course hasn’t finished yet! But leave yourself enough time to find a helpful person at the CC.
Both Foothill Community College (which you mentioned) and Ohlone College (in Fremont) say they allow 9th grade high school students to take courses. However, find out from the CC whether by this summer you would be considered as a (rising) 10th grade student as at least in the case of Ohlone, there are more restrictions on what courses 9th graders are allowed to take; 10th grade is better. Will you also be age 15 by this summer? I don’t see an age requirement on either CC’s website, but some California CCs are twitchier about under-15 kids taking courses. Both the CCs also look like they restrict HS students to taking CSU/UC transferable courses. Also, while neither Foothill nor Ohlone appear to, note that some other California CCs may have restrictions on attendance by high school students from outside the community college district, or may charge out-of-district high school students higher fees.
Be aware that there won’t be as many courses or sections of courses offered during the summer at CCs. I doubt the CCs have their summer 2017 course catalogs posted yet, but take a look at what they offered during summer 2016 to get an idea for what might be offered of interest to you. I don’t see what Foothill offered. Ohlone in summer 2016 offered one section of Math-101C (3rd semester Calculus), one section of Math 103 (Linear Algebra), and one section of Intro to MATLAB.
Be aware that as a high school student, you likely won’t be able to register for the summer courses as early as the regular students can. Make sure to do everything on the HS end and for the CC admission process in a timely fashion, so that you can register for courses the first moment you’re officially allowed to! The sort of courses you’d be interested in may be courses that fill up, so you’d have to go on the waitlist. Try to find out from a CC counselor whether courses of interest to you fill up in the summer, and how easy (or not) it is to get off the waitlist for such courses at the CC in question. Note that being on the waitlist may possibly still work out; for instance, if the course you’re trying to get into is a math course, typically there will end up being a number of students who drop out once the first quiz or exam comes back. Go to all the classes, do all the work, be friendly with the instructor, and hang in there!