You can have a standard plot with standard characters but still have a good show. Nevertheless... I may have the unpopular opinion here but i think Gakuen Heaven is completely dull.
Gakuen Heaven follows the typical formula of "the new dude in school who is fawn over by all his classmates" which is a typical cliche in yaoi/yuri, shoujo and every other anime which involves school romances.
But you can have this setting and still make a great story, but in the case of this anime the plot is boring, all twists are hand-fed to the viewers from the start, and the only one which isn't (which i won't spoil because it happens in the end) it's completely random and pointless. The characters are dull stereotypes who doesn't add nothing rather than being handsome love interests to the main character. And Ito Keita himself is completely weak as the protagonist.
I know that this series, as it's based on a yaoi videogame, was probably made as some sort of fanservice for it's fans... but i expect to have at least a little bit of substance if i decide to take my time to watch a 13 episode show. I kept waiting for something to surprise me but everything was a disappointment in this one. There has been BL anime/manga with the same exact premise that still manage to be really interesting... but Gakuen Heaven couldn't get me engage in it.
But i understand that some fans may enjoy it. It has cute bishounen and a happy-go-lucky storyline (eventhough it's extremely pointless)... so if that's enough to float your boat it will probably satisfy you.
If you are someone who prefer something more substantial, it's a waste of your precious time.