The study was carried out with prospective teachers (PTs) in Primary School Teaching Program in a private university (n=43) and Middle School Mathematics Teaching program in a public university (n=46). In this paper, multiple-case study was adopted to explore PTs' opinions and to reveal the differences between two universities in terms of taking a mathematics education course through online teaching when the COVID-19 pandemic was very first appeared. The content of the course was based on the principles of school mathematics which were put forth by National Council of Teachers of Mathematics [NCTM]. Hence, the principles were used as the themes in the data analysis. For this purpose, a survey was used to explore the advantages and the challenges faced in online teaching from the PTs' perspective. The analysis of the data showed that the PTs at both universities had both positive and opposing views towards online mathematics education course as mandated by the pandemic. Besides ,the technology principle in NCTM was found to be a principle in which the other principles were organized and applied according to it. PTs' reflections on the learning and teaching principles in practice were at the forefront. The reflections regarding the assessment and the equity principle were not among the principles that were frequently mentioned. Discussion on and suggestions for overcoming the challenges were elaborated on.