watch Pet Girl of Sakurasou
and Oreimo
kids making their own manga

based on art, there are characters that do look alike in different animes.
Walkure Romanze
Mio looks like Lala from To Love-Ru (before she cut her hair). Kind of similar personality
Bertille looks like Sylvia from Princess Lover! Very different personality.
but what's so bad about similar art?
it's all about the plot!
if you were able to draw some half decent anime characters in 6th grade,
were you also able to draw out your own manga?
constantly redrawing your characters doing different actions
and making your own story for them?
i took art classes from elementary to high school, and i was able to draw and shade some great looking characters,
but tbh, i doubt i would have the skill to draw my own manga.
i didn't really continue drawing, and i can honestly say that i was a better drawer in 6th and 7th grade than i am now.