Poke: Of course manga can demand for great deal of effort! Just look at Death Note! But I have to say, that compared to oil painting, it has a very simple base. It is much easier to learn and understand (but mastering is harder!) while oil painting has more details you have to note. And manga is rarely colored, which can be easily mistaken as lazyness.
And realism shouldn't even be compared with manga, manga can be any comic that is made in Japan, while realism needs an idea, a meaning and so on. I find it very unfair to demand manga fulfill same demands, it is more wider a concept. Realism can't have manga in it but manga can have realism in it, do you get what I mean?

Parinia: I can explain why teachers hate manga style. BECAUSE IT IS EVERYWHERE!!! Any style will lose its edge if everyone would be doing it, and rare would put any meaning in it (yes, many just like to draw pretty pictures!) Whenever some pupils start to draw/paint something new, they never tend to change their style or try something new. The big eyes are always there, just like the skinny appearance. (That can also annoy many female teachers: you must have noticed the ideal of beauty in these comics! Yes yes, it's part of the style, but so is photoshopping every ad or having sticks in the catwalk! It all changes the way we think.)
Many are also criticizing the mentioned huge eyes. Is it very Asian? No, it's Western (manga was inspired by Disney anyway) and it is sad to know that many Japanese and Asian girls are cutting their eyes to make them bigger, more Western! I don't blame only manga for this, but appreciating the original Asian beauty isn't very visible in manga.
And yes, some teachers even hate the way manga is written: females are bottoms, males are ruling. Even Death Note has this aspect in it. :1

Many have made the conclusion that unlike f.e realism that pointed out the bad things in this world, manga is creating ones. Of course there are mangas out there that are trying to improve things (Gintama tells about abandoning hatred, Naruto about friendships, Bleach about... BANKAI!) but these improvements are mainly thanks to the plot! The DRAWING style is the one causing bad vibes!

God I made a long post. O.O But Parinia and every other manga drawer who feel discriminated in art lessons: I understand you. Teachers should honor stick men too! (I believe that they're one of the best styles ever, it allows pretty much anyone express themselves without the pressure of the looks!) But try to understand what the teachers are facing here. They want you to have a wide aspect in art, they just have hard time fitting manga into it.