GPA plays a factor in the school's decision making process, definitely. Experience which also plays a factor must match the co-op you're applying for.
Knowledge in multiple languages AND software development applications is definitely the way to go. It's not simply about code but also about the tools used to make the coding process easier (and thus speed up development time).
C++, C#, Java, C, and even knowledge of web development would greatly enhance your ability to get the co-op position. Lastly, try to research about the co-op your taking.
@Little Captatin, thanks for your helpful suggestions.
Based on your experience, do you know any useful website/books/other resources that can help me to know what needs to be actually done in the work place ? I mean not just the coding, but for example how to speed up software and the process of compilation, and other stuff about problem solving and etc.