Actually, there are discoveries made by amateurs. Last year a high school students, just like you, discovered a new pulsar through one of these programs and a young woman (not a scientist) in Europe (Sweden, I think) discovered a new object in one of these galaxy zoo projects. 

Galaxy Zoo does have many, many people looking at each galaxy, so there is control that way. This <a href="http://openobjects.blogspot.com/2013/06/were-all-looking-at-stars-citizen.html" rel="nofollow">blog</a> describes what has been done with citizen science from scientists' point of view. 