the title says it all. I'm still a university student studying IT. Whenever I'm doing coding, I will frequently look back on codes that I've done before or looking for codes online or on reference book on how to do certain task like using classes, HTML stuffs and such. Is this behaviour acceptable on working place or do I need to learn to code without looking at references?
The nature of this particular beast - computer programming - is so vast that you will never be without the need of references, documentation, examples, and past work.
As you gain experience you will naturally need to look back less often for some tasks, as you come to memorize and gain insights into fundamental orderliness in various systems and topics.
You will eventually come to "chunk" things differently as you move towards being an expert and you'll be able to recall and reconstruct ever more complex and involved sets of codes and varying approaches, but you will always need references because our computer languages themselves are not entirely consistent.
And I must fully agree with pytyx's answer, that getting better at looking things up is an important part of the job.