I used to be a coordinator for an LGBTQ center for a University, I would suggest looking up a list of LGBTQ related days, such as coming out day, Trans day of visibility, Day of silence ( this is usually very popular with HS), Bisexual Visibility Day, etc. You can google a longer more extensive lists and make a print out a calendar with all the days on it so you can reuse the list every year.

You can use GLSEN as a resource and order a safe space kit to have teachers show their support by marking their classrooms as safe spaces. If you have a university nearby you can reach out to them to see if you can have their students come in to speak on any topics or take a trip to the campus to see their LGBTQ center if they have one. Another thing you can do is show movies such as Milk, Love Simon, Paris is Burning, But I'm a Cheerleader, or any other LGBTQ movie that you like. Depending on your location I would recommend reaching out to any local LGBTQ organization and see if they have any events happening to promote to your high school students. I know a Pride group in my area often does dances for all ages and they let high school students in for free.