Good Morning,
What education you require depends on the trade that you are wanting to join. Many people in the Reserves join while still in high school, but a Medical Officer in the Reserves for example requires more education than someone who wants to be a Supply Technician, Infantry Soldier, Boatswain or trades of those natures. It really depends on what trades interest you. If you're unsure your best bet is to go into your nearest CFRC and speak with them about the available trades in conjunction with your education.
If you join as a Reservist and are going to an approved post-secondary education you can also apply for Reserve Education Reimbursement. If your academic program is approved, the Canadian Armed Forces has the ability to pay up to $2,000.00/year to a career maximum of $8,000.00, which is another reason why people still in high school join - this is an opportunity to lower their tuition costs for the post-secondary institution that they eventually attend.
To clarify on the first summer in, depending on what unit you join is dependent on whether you can do Basic Training on weekends leading up to the summer, or whether that unit has summer only training. Just like Basic Training, the requirements of how much you have to train is dependent on your unit. Most Army Units I have been involved with train one night a week and a minimum of one weekend a month. The Navy Units I have been involved with trained one night a week and only one weekend per every 2-3 months; this is something to discuss with your Recruiter once you know what trade you want to be (as that will in most cases determine which unit you need to apply to).
Your best bet may to find out what Reserve Units are in your area and what trades they are looking for. There is no sense in deciding you want to be Artillery if there are no Reserve Artillery Units in your geographical area.
Reserve courses are schedule around the school semester so your full time training will not conflict with your school studies.
To find your nearest Canadian Forces Recruiting Centre please goto the following link: http://www.forces.ca/en/centres/findarecruiter-110
Best of luck in your journey.