[@Rhiannon]
Well, I guess I'll have to say a bit of detail about my OC. I take the inspiration from the card game shadowverse, specifically the class shadowcraft, which consists of undead creatures and uses necromancy for its powers. Skeletons are extremely common there, although only a few could speak, while others just roar, for the audio purposes (those that can speak for themselves, I don't know how they do that). The OC I'm using is, yeah like you said, reliant on the necromancy power that animates him. But I think I'll make his necromancy like the human stamina, which he could self-regenerate using the necromancy he has (that is pretty darn difficult and complicated to explain how he does that). For emotions, I could probably handle with that, as I have written about the Strike Witches anime, specifically about the Neuroi faction, a faceless and emotionless creature. Also, I also thought about using a kind of flame that is created using necromancy that burns neatly inside his skull, that acts as his eyes (although without irises) and one of his ways to convey simple emotions (For example: cyan for normal, red for anger, yellow for excitement, and deep blue for sadness, and none when he is sleeping). Complicated emotions, like [@Pathfinder] said, he could exaggerate their movements, like when disappointed, he could jerk his head down, or facepalming, or when frustrated when his classmate could not understand, he could just slam the table or something.

For speech, it depends on how the world works. If it has modern technology like we do, he may use a type-writer then let a computer say it for him (like Stephen Hawking, but this time he has to type what he wants by hands). It has all kinds of utilities, including Wi-fi (wow...), so I assume they are very modern, but in case they have yet to create machines like that, he can write down on notes (Like Eu in the Kore wa Zonbi Desu ka? series. She can speak but uses notes because of reasons). His emotions can also be written in forms of notes as well, similar to chatting.

I might make him able to speak, but on a very harsh condition, and in normal conditions, he cannot (like consuming a huge amount of necromancy). The lack of communication can also be used for comedy purposes, like misunderstandings or so. 

I like the idea of undead creature (sorry if I sound like a freak), and would like to use it for the RP, if you allow.
Thank you for taking the time to work this out with me.