Lol, when we split atoms, the mass still stays, it's just the energy holding the particles together gets released. Mass and energy are 2 separate things.
Fission in the splitting of atoms(releasing the energy in the particle bonds) and fusion is the combining of particles into atoms.
As said the Law of Conservation of Matter and Energy say that matter and energy can not be destroyed or created, but rather changed, but not into each other.
Pay attention more in class, you may learn something.