This is really interesting and my immediate reaction is that it sounds like a cool idea! I think that if I'd had the opportunity to go to an LGBT high school that I probably would have. My friends knew that I was gay, and once people start gossiping, the word naturally spread to other people. While I can't remember being bullied for being gay in high school, I also didn't feel like it was an environment in which I could be completely open about it, either.
This led to me finding places outside of the school setting where I could be with people like myself, and do the social things that other kids my age were doing, like dating, making friends and going to parties and outings that were relevant to my sexuality, which is a big part of growing up. For me, school was just the place I went to take classes and as soon as the day was done, I'd immediately take off to be with my gay friends whom I felt most comfortable with. I think that being able to experience this sort of thing within my school environment would've made my school experience so much more than the job it was for me, where I was basically just punching the clock and not getting involved in any sports or activities.
I partially agree with you Meta, in that I think it's very important to build resiliency in youth. They never know what they're going to encounter later in life and it's good to be prepared for the realities of human interaction in the world at large. With that said, I feel that there are other ways to build resiliency than throwing kids in the proverbial shark tank. What brought out those skills and developed that resiliency in me, was being able to find myself and really develop 'who I was', in a friendly and supportive environment which included gay peers and older LGBT role models who encouraged me, taught me, and helped me to develop in a positive way. This didn't exist in my school and it's too bad that it didn't, because not all LGBT youth are going to be as lucky as I was to have the means to seek out their own supports. The benefit of the school wouldn't JUST be to isolate kids from LGBT based bullying, but ALSO to allow them to develop socially, which would build the strength in them that's so critical to their success and survival in the world at large.
On the other hand, I see potential issues with segregation too. Part of what has advanced the LGBT movement so significantly in areas of the world in recent years, has been the increased visibility of LGBT people in society. For youth to grow up to be accepting of us, it's important that we're visible and part of their lives so that people know us as the friends, neighbours and everyday community members that we are. As Gravity has suggested, segregating our respective populations might well end up being counter-productive to that cause, thereby perpetuating the hostile environments that we're trying to remove these kids from by putting them in an LGBT-only school. This brings us to Tlek's concerns about the school being targeted and I think that reducing that potential for danger is probably best served by not segregating the populations.
At the end of the day, I think that so long as this kind of school becomes the exception and not the norm, it might well turn out to be a good idea and a positive experience in the lives of many LGBT youth. I think that it's equally important to have LGBT components to the regular school system, which would include stand alone courses as well as inclusion in the regular curriculum. Kids need easy access to LGBT youth groups or gay-straight alliances in the school setting so that kids not attending these LGBT schools don't have to search for the supports that they need.
If you're an LGBT youth in Canada looking for an experience similar to something you might experience at an LGBT school, check out Camp fYrefly. It's run by some of the amazing people who supported me when I was growing up! 
