Whooooa, a steampunk game! I'm totally digging the theme -- and the unique art style! The coloring and character designs are absolutely gorgeous, I have to say.
1. Is the general plot appealing to you? Is it boring, interesting...?
I don't traditionally find school-based plots to be super interesting, but I think the steampunk qualities will make it feel fresher and more unique than your usual school setting.
2. Is there something specific you would like to see in a BL visual novel?
Lots of hot bachelors, obviously, but you got that on lock already. The top/bottom option isn't really something I care about -- I prefer just to see lots of character/relationship development so that when sexy stuff does happen, it feels natural. (which is easier said than done o.o')
3. What do you thing about the multiple endings and the possibility of changing to another character's route? Is is too open for your taste?
A little bit open for my personal taste, but I just tend to prefer more linear stories where I don't get confused or worried I'm gonna get the wrong ending. I think it's a neat feature and, as long as there's a guide, I don't see there being any problems.
4. Do you like the characters so far? Which one do you like best?
I'd have to say Biel or Tirs. The main character is just so fabulous in his coat, and those green eyes...

His design is awesome!
Biel looks pretty cool too though, and I have a soft spot for more gloomy/quiet characters. Definitely would be first on my romance list~
5. Any suggestion or criticism?
Please please please have a sex scene on an airship. Steampunk Mile-High club, yes please
In all seriousness though, it looks great -- curious about how long you're planning on making the game though! Is it a short VN or a longer one?