Is a person considered sinful or blameworthy if a female student kisses him as a congratulation for a result, given that he did not expect it and did not stop her but thanked her?

Perhaps the girl kissed you due to her ignorance of its prohibition in your religion. The blame here falls on you and on Muslim youth in those lands for not adhering to their religion and demonstrating its rituals. There is no sin upon you as long as you did not intend the kiss and did not approve of it, in accordance with His, the Almighty's, saying: "And there is no blame upon you for that in which you have erred, but [there is blame for] what your hearts intended." However, you must explain to her that this act is not permissible in your religion and that you will reject it in the future.