There is no yaoi or shounen ai tag because this is neither of those. If you think this is either of those then you obviously don't know what yaoi and shounen ai actually is. The synopsis and crossdressing tag should be enough to figure out that is going on.
This isn’t yaoi, yaoi is hard core BL with sexual elements, smut basically. This isn’t that. If they actually do end up in a relationship it may deserve a shounen ai tag, but even that requires abit more before it gets the tag. A slight attraction between two dudes isn’t enough. If they start dating and kissing or crap then it becomes shounen ai.
If a slight attraction between two guys is your def of yaoi then that would make any series with a trap character that someone blushes over yaoi.