Follow the informal pathway as far as it goes
The system is set up to work for everyone. Obviously, you want your rights respected. Of course, anyone else involved wants the same. That's why progress has to be step-by-step, so that no one else gets to decide things without everyone affected having their say.
TIP: Let the person you spoke with first know that you're taking the next step. This is a courtesy (you'd want to know in advance if someone was going to complain about something that you did or decided, right?). It's also pragmatic: The other person will then start collecting their own info early, so, when asked to respond, the answers will be ready. That could get your problem solved faster.
Reality check your expectations
Knowing how others hear you may help you to re-think your goals. Reflect back on your conversation with an open mind. Do you now better understand the other person's point of view? Are you sure that the resolution you have in mind would seem to be fair all around? Is it still necessary?
TIP: Don't let your ego waste your precious time. Sometimes we all can get a little invested in how right we are. But you'll want to think through the costs-in energy and emotional attention-of pursuing a concern as far as you could. Is it worth it? If so, go for it. If not, you don't need to make your choice not to go further feel like a defeat. After all, being practical is a sign of maturity.
If necessary, review the formal grievance policy
When all else fails, there is more that you can do. Our system protects your right to fair treatment, especially regarding problems that have severe consequences for you. There also is a Student Grievance Officer who can help you to go beyond the informal route and understand our formal grievance policy.
Prepare yourself for a major effort. The formal grievance procedure is somewhat legalistic and typically would require you to devote a lot of time and gather (or write) quite a bit of documentation. That's why it's important to do everything that you can to resolve problems informally first.