there are flashbacks showing it , when the third had his flashbacks of everyone. be more observant.

why wouldnt it fit into the time line? jiraiya was taught by 3rd when he was 12 say 10 years later he teaches 4th who also is 12 at the time 10 years later he teaches kakashi who is 6 at the time, 20 years later he teaches naruto. and wow the time line fits, what a shock.