Please keep it simple so that a grade 11 kid, new to nuclear physics would understand.
We all were once new to nuclear physics and then to particle physics that evolved from nuclear physics.
Basic rules in physics are classified into conservation laws . Energy is conserved , the sum of all energies is conserved in an isolated system, as well as momentum. It was thought in classical physics that mass was also conserved, but this proved to be wrong at the level of studying the interactions of nuclei , of which all macroscopic masses are composed. This led to the mathematics of special relativity , where particles can decay to lower mass particles.
Conservation of energy and momentum still holds in special relativity, and the decays of particles seen in cosmic and laboratory experiment led to the necessity of defining an electron neutrino, as well as two other neutrinos. Here is how it was proposed and then discovered: energy and momentum would not be conserved in the decay of the neutron to a proton and an electron, it seemed that a neutral particle was taking away energy and momentum. So they defined it as an electron neutrino.
Then other particles were discovered later , like the muon and the tau leptons , also necessitated the existence of a muon neutrino and a tau neutrino. They could not be the same because to explain the the decay of the muon to an electron, one needed two neutral particles, an electron antineutrino and a muon neutrino. Thus rose the concept of lepton number conservation, : an electron cannot just disappear or appear (as is the case in muon decay, which leads to the world of antiparticles:
For every particle in the particle table, there exists an antiparticle, which has the opposite quantum numbers, for the electron it is the positron.( for the proton ,which is composed out of quarks, the antiproton). The positron has a negative electron number, and when they meet they disappear into two photons. That is the way for lepton numbers to disappear. The anti electron neutrino carries a negative electron lepton number . The antiparticle mathematical world is the same as the particle world with characteristic quantum numbers in the negative, so when particle meets antiparticle they can disappear.
Otherwise and electron cannot disappear, and when created, as in the muon decay, an antielectron neutrino has to appear.
All this is the result of a huge number of experiments which led to the standard model of particle physics, which you may study if you continue into physics in college.