When you are referencing that material (the blueprints) you are recreating one environement into another form. You are building it from scratch in your own style according to your own creative patterns, adding architexture, spawn locations, game devices, rules, setting lighting patterns. You will develop it for improved gameplay for this game. This is all your own work.
People here reference shacks and 1900 era western mining/outpost buildings from flickr. This is the same. They are recreating them how they see fit, in their minds, for TF2. You would provide gratification to anywhere you took a prefab or model from, created by another person. But changing a mode in an already produced map doesn't make it your work and you have no right to release as such.
Recreating every brush from scratch as to how it was before will not make a difference. The gameplay was designed by the level designer and although changing the mode may likely change the gameplay the environment is still not yours to edit.
I'll try and answer your questions more directly, from my own perspective.
Your school map:
This took no thought, no inspiration to make.
Yes it did, you saw the layout and you were inspired by it, you converted it to work for cs (much as i had mentioned in the first paragraph). You used released public developer material to produce a unique environment never before seen ingame. A lot of games are based on places in reality, that doesn't mean that the level designers shouldn't get credit for producing them.
Yet wasn't it an act of creation?
Yes it would have been, you applied the necassery functions, you provided people with an environment to the level of skill and knowlegde you have acquired.
I'm not creating something unique. Yet can't I call the map my own?
You can, just as other games can call their immitations/replications of places in reality their own.
Try applying this to say (since we are all referencing hydro) Hydro:
This took no thought, no inspiration to make. Yet wasn't it an act of creation?
No it didn't, someone else created it, you just flicked a switch to a lack of better words to change it's mode. It just happens to be that TF2 has more hands on needs to do such things.
I'm not creating something unique. Yet can't I call the map my own?
The map was already created for TF2, no you cannot call it your own.
This gets a little more difficult to define when dealing with "ports" of maps from other games. This area is a lot more grey.