A Matter of Timing
First, sorry for the necro, and a disclaimer... as Dennis Miller has oft said, this is just my opinion, I could be wrong.
Theory - Negi Springfield is/looks older than we believe him to chronologically be.
1) Something that everyone is assuming is that in this fic Negi is the same canon age that he is in most fics. This may not be the case, as it is not specifically stated that he is. In fact, people have not made any comment of his age at all, which leaves open the possibility that Negi could be physically or chronologically older than canon. Let just say for the sake of argument that he may be the same age than the students he will be teaching at Mahora. After all, you could always set Negi's birthdate back a few years without affecting the present day story.
2) Negi did not have any undue reactions from females until he was leaving for his new job in Japan. It may be possible that the powers he got from Morrigan Aensland were latent until he reached magical or physical maturity. This lends credence to the theory that his powers weren't apparent until now, as I doubt that Negi would be sent to teach at an girl's school if he was having the problems with females that he has now.
3) Evangeline at first glance mistook Negi for his father Nagi. It may be possible to mistake a 14-15 year old for an adult, especially of you haven't seen that adult in a while, but it is almost impossible to mistake a 9 year old for the same.
A question that needs to be settled is who exactly is affected by Negi's (presumably) incubus powers? Any female? Any female attracted to males? Since we do not have any confirmed lesbians in the story, it's hard to tell how extensive his powers may be.
Another question is whether Negi will be able to control his powers. Presumably, the Headmaster will be able to recognize and assist Negi once he manages to get to him, but since the power is a relative new magical ability of Negi, it may be subject to fluctuations that he would not be able to completely suppress.
Lastly, what will be the reaction of females who have been affected by Negi's power once he manages to control it? Will they be angry that they acted the way that they did? Will they have residual attraction to him? Will he be imprinted on them permanently and they won't be able to not be attracted to him?
The reactions of all the female story characters to Negi is rather amusing, but what I want to see is Eva's once she realizes who he really is. Of anyone, she's probably going to have the most intense reaction to the new teacher.