That's excellent that your students were rewarded for their hard work. It's not uncommon for students to underperform on their first real SAT. For many, they begin to second-guess all their efforts, which is why I always recommend that families have their student sit for these tests at least 2-3 times.
As for any month being easier, that is a big myth and totally not true. I know you may have spoken to another SAT coach, but I am part of a community of 100s of test prep professionals, and this is one thing we always get a nice chuckle about when we hear of it.
However, I think it is fantastic that you have targeted the March SAT as their next test. Typically, I like to condense test prep into the shortest window possible, which is why it is essential for families to consider their student's readiness, their calendars, and which months of the year are optimal for intensive preparation. It's not uncommon for me to recommend delaying test preparation for a student based on their individual needs, since test prep is such a huge commitment.
For that reason, you want them to gain confidence through steady progress and eventually achieve their goals. From there, they can move on to other productive activities to help bolster their college application resumes.