Imam al-Shafi'i (may Allah have mercy on him) says that the imam has the choice: when he wants to teach those following him the prayer, he may pray in a raised place to show them. Once they have learned the prayer, the imam and the followers should pray level on the ground.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5231