Complex sentence: During the "seconde", students mostly take the same courses, despite having different intellectual and academic skills and interests, so it is usually thought to be an easier year than either the "première" or the "terminale".
Split into simpler sentences: During the "seconde", students mostly take the same courses, despite having different intellectual and academic skills and interests. Therefore, that schooling year usually is deemed easier than either the "première" or the "terminale".