Recently, on my school, I made an ethernet cable from an used one (2 meters distance) and I thought to share my internet connection with an other Windows PC (or at least use my PC as a pass-through ...

If by "ethernet is unchecked in network control panel" you mean it's unchecked in the Network Port Configuration list, go ahead and make sure it's checked. It won't show up in the Sharing pref pane as ...