Adeyo Samuels has a lot of great ideas. Define your population, sample, and size. Bigger is not always better. Fewer, more indepth questions and a follow up is better.
All unstructured/semistructured interviews or first focus groups? Decide on what you want to know. All these issues go back to your aims and methodology (e.g., a grounded theory study will be vastly different than a phenomenology study, etc.)
By perceptions, what do you want to know? Look both at the COVID-education literature and the broader field.
Pilot your interview questions after carefully crafting them [can include initial analysis of the practice interview to see if you get the information you want and if questions are clear]. Consider techniques to improve the questions, such as the Delphi technique, interviewing/reviewers of outside experts, etc. Chenille has other ideas. I will review your guide if you asked.
Here could be the simple way to start. "Tell me about teaching during COVID." Then ask probing questions and inquire about best/worst experiences and "tell me more." Avoid why questions!