Dear [Name],
Thank you so much for your kind offer to have me stay with you during your visit from abroad. I truly appreciate your hospitality and generosity.
Unfortunately, I must politely decline. While I would love to accept, as a student in school I have a lot of exams coming up and must dedicate my time studying. Additionally, I have a part-time job that I need to continue working at to help support my studies. I also have a number of extracurricular activities that I am involved in that would make it difficult for me to stay with you.
Once again, I am grateful for your kind offer.
Sincerely,
[Your Name]