I don't see anything wrong with mandatory community service in order to get a diploma. School is about learning, and community service offers some very important lessons. Maybe "volunteering" is the wrong word for it...but really, students are FORCED to learn math, english, science, etc. while in school, what is wrong with forcing them to perform community service? If it was one school, or a small group of schools who were like "hmmm we need some painting done, and some repair work around our schools...let's force all the students to help or we won't let them graduate" then I wouldn't agree...but this isn't that.
Things change, requirements for graduation change. When my mom was in highschool in Newfoundland it was only 3 years long (grades 9 to 11)...now it's four (grades 9 to 12). This mandatory community service is the next change for the education system, it will add a new layer of things for students to learn. It's a general improvement in the system and I like it.
I graduated before the community service requirement was introduced in Ontario but I would have been able to complete it no problem...I worked full time during the summer from the time I was 16 so it would have all happened during the school year, and I had a part time job during the school year from the time I was 16 too...that said, I still spent a lot of time being completely doing things that were in no way productive...I definately could have found the time to volunteer for a meer 40 hours (or even 100) over the 4 years of highschool.