So far (2019-04-27), the following schools have confirmed their participation:

Austria Höhere Technische Bundeslehr und Versuchsanstalt, Rankweil
Belgium Institut de la Sainte Union, Kain
Bulgaria Lycee bilingue “Romain Rolland”, Stara Zagora
Czech Republic
Gymnázium Rokycany
Denmark
Kalundborg Gymnasium, Kalundborg
Estonia Mustamäe Gümnaasium, Tallinn
Finland
Oulun Lyseon Lukio, Oulu
Germany
Geschwister-Scholl-Gymnasium, Sangerhausen
Hungary
Kanizsai Dorottya Grammar School, Szombathely
Italy Conservatorio SSma Annunziata, Empoli
Latvia Gymnasium; Gulbene
Lithuania
Vida Sinkeviciene Saule Private Secondary School, Vilnius
Luxembourg
Athénée de Luxemburg, Luxemburg
Malta
G.F. Abela University of Malta Junior College
The Netherlands Theresialyceum, Tilburg
Romania
Colegiul Naţional “Moise Nicoară”, Arad
Portugal Escola Secundaria Alcaides de Faria, Barcelos
Slovakia
Gymnázium Jána Hollého, Trnava
Slovenia
Gimnazija in ekonomska srednja sola, Trbovlje
Spain
IES Narcìs Monturiòl, Figueres
United Kingdom
Aquinas College, Stockport

Still waiting for confirmation from… 😉

France
Greece