Let’s say you have an awful teacher in school that doesn’t know how to teach properly. Are you allowed to tell your parents that that teacher is bad? Or is this Lashon hara? If it is Lashon Hara, how are you supposed to explain that your failing the class because the teacher is bad. Please cite sources.
There are seven conditions for one to be allowed to say "lashon hara" for a constructive purpose. Here they are (from here) with my thoughts on how they apply to your situation
The information spoken must be completely true and witnessed or verified by the speaker: applies in your case since you are reporting first-hand experience
The issue must be a problem (e.g. transgression, relevant character flaw or bad behavior) from an objective viewpoint, not merely a preference or sensitivity: this is where it is important to relate the facts as they are and not your opinion or feelings about those facts
The speaker must first rebuke the subject directly, in a kind and gentle way which is likely to have an influence. (If the subject will not listen to any rebuke, or if trying to rebuke him can make the Lashon Hara ineffective, refer to ch. 10 in Hilchot Lashon Hara and ch. 9 in Hilchot Rechilut for the Chafetz Chaim’s treatment of the subject.): it is unlikely you can influence the teacher to change
The information cannot be exaggerated or embellished, even if it’s the only way to get the listener to heed the information
The intention of the speaker must be purely to help in the situation, not to degrade the subject or cause him shame: your only goal should be to find a solution, not to get the teacher punished/fired/etc.
If the constructive purpose intended by the speaker can be achieved in a way other than speaking Lashon Hara, the speaker should resort to that other method: speaking to your parents first appears advisable before trying other things on your own
Any damage that is caused to the subject as a result of the Lashon Hara should not exceed that which would be decreed by a Beit Din (Jewish court) if the case were reviewed there. This is difficult to evaluate, so that situations that impact the livelihood or other areas of the subject should be referred to a Beit Din: you would have to be reasonably sure that consequences from your discussions with your parents won't result in the teacher being blamed or fired
So it appears you can discuss with your parents the behavior of your teacher with fact-based examples you have personally observed, without embellishment. You should avoid qualifying the behavior with your own views (e.g., awful, doesn't know how to teach) recognizing you might not have all the experience required to judge. Your goal in doing this should be to get help to improve things and not to get the teacher punished. Your parents can then work out a plan with you on the proper way forward. Good luck!
This is an interesting question, esp. regarding young kids who are not experts in the halachot of Lashon Hara.
There is a concept of to'elet - "useful purpose" - which allows one to disseminate information. I'm inclined to think that if your teacher has harmed you in some way, then it becomes useful, permissible, and sometimes necessary to tell your parents. I know a few boys in my high school that were sexually / emotionally abused who didn't tell anyone possibly because they thought this was lashon hara. That's really sad.
Failing a test or a class may not necessarily be the teacher's fault. If the teacher is "bad", that can be a child's perception of things created only because s/he is failing and the base result could be a lack of effort, understanding or other problems not caused by the teacher. It is important to listen to your child's problems, but more important is getting to the real cause of the problem. If the teacher is not at fault, then, of course, future similar reports sound to me as if they would be a form of lashon hara, or possibly a lie.