STAPHYLOCOCCUS AUREUES or BACILLUS CEREUS (from a mixed culture with e. coli)???

Based on the original information (gram+, rods, spores), the original unknown was Bacillus. A very common contaminant in bacterial culture is Staphylococcus spp., so you are probably correct that you picked up a Staph contaminant. If you can get back to the original culture, you can start over and get a pure culture. Otherwise, your teacher's advice may be the only thing to do and go forward with you Staph culture. Also, it is possible that there are some Bacilli left in the current culture and so you can try plating the culture and looking for isolated colonies of something other than Staph, but that is a long shot. If you can, go back to the original culture, otherwise you just have to forge ahead with what you have. Good luck!