We're not talking about Thai education standards. International schools deliver education following the standards of the country which gave them full accreditation.
Sorry, but it's a uninformed comment. People on guardian visas enroll their kids in international schools which are quite good and expensive. We're talking about 7K-15K USD tuition per year for one kid.
Is it possible to write invoices without having registered as a business in Sweden? If it's the case, than just explain it in a cover letter. Attach recent invoices and highlight the corresponding transactions on your bank statement.