You may skip MA103 as well as MA104 etc. One of my friends started in the Yearling math class because he validated and did well on the FCE during Beast.
You needed an 80% to pass it last year. I don't know where you got the 90% from but they may have changed it.
And you are right this for this year too AF, from the Mailing on Math:
Early in Cadet Basic Training, the Department of Mathematical Sciences
administers a “Gateway” exam to assess fundamental concepts. Therefore, we encourage
incoming cadets to evaluate their mathematics skills and work at remediation of any
deficiencies prior to arriving at USMA.
The Department of Mathematical Sciences administers two additional “Gateway”
exams during the first semester in MA103 (Mathematical Modeling and Introduction to
Calculus). Self-paced texts and software files offer opportunities for self-remediation.
Failure to pass at least one “Gateway” exam at mastery level (≥ 80%) may result in
placement in MA100 (Precalculus) during the second semester instead of continuing into
MA104 (Calculus and Differential Equations), or other scheduled remediation. Cadets
enrolled in MA100 who do not demonstrate proficiency in these fundamental mathematics
skills by the end of their first year may be separated from USMA.
Almost any high school algebra book is a suitable reference for these fundamental
concepts.