Please ask the biology teacher to pick up a good aquarium book - The Simple Guide to Freshwater Aquariums by David Boruchowitz would be an excellent choice - so that you can both learn about the nitrogen cycle and other issues:
a) Your tank is not ready to support that many fish that soon - which is why you have detectable ammonia.
b) goldfish and tropical fish do not do well together - the goldfish ideally need 65-68F rather than the 75-77F the tropical fish need.
c) As was already stated - an oscar needs a bigger tank. eventually the oscar will have health issues in a 55g tank.
d) a new 55g tank doesn't have enough algae to support 4 "sucker fish" - if these are common pleco's they will need to be fed AND will outgrow the tank (they get 18" long) - they will create more algae than they eat. If they are other "sucker fish" - ie "chinese algae eaters" they will get very aggressive as they get bigger, and most likely decide they prefer fish slime to algae...
Your biology teacher may be able to get some guidance from a local fish club - my club sponsors a number of tanks in schools, for example.
__________________
--------------------
Just "Red" 
(Paulhus is my lastname  )
|