Any experience with preferred names and school?

[Silas]

I'm going into 10th grade, school starts back next week. (agh XD)
First semester of 9th grade, I didn't say anything but "here" during roll call -- for us, they call your name, and if you go by something else, you say it instead of here. Frederick is Willie, John is John Reid, Victoria is Tori, etc. I got the balls to request "Chris" in second semester. I want to take it a step up and go by Christopher, as it is my preferred name and I really dislike "Chris". It reminds me of my druggie uncle. -_-; lol. I already write Christopher on most of my papers. @_@; But they call me my icky birth name and I have no idea who they're talking to.

I'm sorta nervous about it, though. >_> Except for the Spanish teacher. I think she likes me, and I plan on explaining my gender to her so she won't think I'm stupid for using the "masculine" form of words.

Anyone have any experience requesting preferred names?
Especially if they're no part of your birthname. Most teachers here say they won't call you something that can't be derived from any of your legal names (first, middle, last). Christopher is not part of my first name at all. XD Chris is, but I want to be even more confident this year.