Mizuiro Jidai

Both these anime are school life romances. The characters are both in junior high and learning about love, friendships etc  

Both are serious romances, featuring little comedy, and lots of drama. Both are in a school setting Both feature characters learning about love and it's struggles Both take time to develop and set up a proper story that pays off in the end I personally got big Kimi ni Todoke vibes while watching Mizuiro Jidai, Mizuiro Jidai felt like a middle schooler version of KnT, with a but less drama. 

Mizuiro Jidai might be a older anime but it deals with middle school love and school girl problems just like this show both having a leading girl starring in them.  

Both are slice of life, shoujo anime and both are 90's anime. Only difference is Jidai's characters are middle schoolers with some growing pains and Marmalade Boy's characters are high schoolers. I highly recommend watching both anime if you like shoujo.