Is it permissible to buy a high school diploma for someone who studied the third year of high school but was unsuccessful and suspended from studying? And what should be done if she bought a second year high school diploma?

It is forbidden to buy and sell forged certificates, because it falls under the category of bearing false witness, and it involves cheating the nation. Whoever buys a forged certificate must repent to Allah and refrain from working with it. And whoever fears Allah and gives up something for His sake, Allah will compensate him with something better.