One of the few yuris i've read back then
http://www.mangago.zone/read-manga/i_fell_in_love_for_the_first_time/
15 sai is really good, so as the reconmended one anonn give. try reading this
http://www.mangago.zone/read-manga/omoi_no_kakera/
http://www.mangago.zone/read-manga/hoshikawa_ginza_yonchoume/
http://www.mangago.zone/read-manga/sweet_guilty_love_bites/
http://www.mangago.zone/read-manga/my_daughter_s_girlfriend/
my fav is still this one.
http://www.mangago.zone/read-manga/citrus_saburo_uta/
try this maybe, it's very good and small age difference http://www.mangago.zone/read-manga/octave/ - See more at: http://www.mangago.zone/home/mangatopic/1201979/?cid=1204971#respond
I just recently got into yuri and I am looking for something like 15- sai. I like the age gap theme! Please help <3