I'm sorry about what happened at the school assembly. What a terrible idea that was on the speaker's part!
If there were at least a few dozen people at the assembly, rest assured that you weren't the only non-normative student there. Depending on which study you consult, somewhere between five and 10 percent of people are non-normative in some way--gay, lesbian, bisexual, transgender, etc. So for every 100 people at that assembly, five or more of them probably do (or will) self-identify as non-normative.
But that age range, a lot of people are still confused about their sexual or gender identities--they may consider themselves "curious," for example. And many of those that have begun to self-identify as non-normative don't come out to classmates for fear of becoming social pariahs or even victims of violence; though things are better for non-normative teens than they were even a decade ago, it can still be a tough road and not everyone is willing to take that road while they're still in high school. So there aren't many teens who have both begun to identify as non-normative and are willing to be out to their entire school, which is likely why you were alone in walking out to the floor.
Many people wait until they've graduated from high school and left home before they come out or before they even to start to explore their feelings. Some people deny or repress their feelings for years on end, and have a much harder time down the road when their true feelings erupt to the surface.
As for NDK, we welcome everyone, both normative and non-normative. Our anti-harassment policy helps protect anyone who's non-normative in some way (sexual, gender, or other) from being harassed by other attendees and we take it very seriously. We also have a number of panels related to such topics at the con each year (the numbers and variety depending on submissions from attendees, of course). Quite a few attendees (and even staff members) identify as non-normative in some way; though I can't prove it, I would imagine it's a disproportionately large number given the general level of acceptance toward non-normative identies among anime fans. In other words, we hope you find NDK a safe haven where you can be yourself and spend time with other people who share your interests and have similar life experiences.
EDIT: Oh, and you may want to join the Gay/Straight Alliance at your school (or start one if there isn't one already). Perhaps some of those non-normative students who are too uncomfortable or afraid to come out to the entire school would be comfortable enough being out to a select and supportive group of people.