Promoting directly from within your own ranks is the absolute best way to fill lead teacher vacancies. Your current assistants already know your center's unique culture, they understand your software systems, and they have established trust with your enrolled families. However, you cannot just hand an assistant a clipboard on a Friday and expect them to lead a preschool room perfectly on Monday.
You must implement a formal, structured mentorship program. Have the assistant slowly take over specific classroom elements one week at a time. Let them lead morning circle time first, while the current lead teacher stays present and supportive. The following week, have them take over drafting the digital daily reports. The week after that, coach them through face-to-face parent communication during the afternoon pickup rush. Give them the operational tools and the lived confidence to succeed long before you ever officially give them the title. Teachers promoted this way succeed at a dramatically higher rate than those hired externally.