This is an example of Present Perfect Tense: a past action with some present connection. At some time in the past I started being a student. I was a good student, and I have continued being a good student up to right now- the time I am speaking to you. What happens after this statement not mentioned.
There are two possible contexts to this statement:
1. The speaker will continue to be a student, and hopes that that past action will somehow influence his/her situation in the future as in: I have been a good student, so you should not accuse me of being lazy.
2. The speaker is no longer a student as of the time of speaking, as in: I have been a good student, so I hope you can give me a letter of recommendation.