thank you for your mail 😊! Great news that 4 teachers will apply!
>Our flight can be from Porto Airport to Helsinki,
> but the return flight must be from Tallinn?
Helsinki and Tallinn are twin cities today. A fast ferry brings you over the Finnish bay of the Baltic Sea in 1,5 hrs. The distance is about 80 km only. The return ticket costs about € 50. If you attend to a 5 days course Helsinki and Tallinn combined, you will transfer from Helsinki to Tallinn on Wednesday evening.
You have many choices for your flight. You can fly to Helsinki on Sunday, transfer to Tallinn with the course on Wednesday, and return from Tallinn to Helsinki by ferry again to take your departure flight. Or you can fly to Tallinn on Sunday, take the ferry to Helsinki, transfer to Tallinn by ferry on Wednesday, and take your departure flight from Tallinn.
Or, if there are cheap single flights for you, you can arrive in Helsinki and depart from Tallinn. Then you need only one transfer by ferry, which costs € 25 – 30.
BTW, hotel rooms in Tallinn are about 50 – 60 % of the prices in Helsinki.