Declare custom structs

In traditional development defining structs depends a bit on the architecture of the software and certain conventions but I'll neglet this in this post.

To give you a simple eosio-sepcific answer:

1. You don't declare structs in the Abi yourself, the eosio.cdt generates the Abi for you and includes the types that are needed to be included. (There are some special cases where it makes sense to manipulate the generated abi but you shouldn't care about that)
2. You typically declare and define custom structs in one of your header-files (in your case you probably only have a single one).

Simply do:

    struct student_t { 
        uint id; 
        string content; 
        bool present; 
    }

somewhere at the top of your contractname.hpp (within or outside of the contract-class).