Can a 14-year-old freshman play high school rugby?

Yes, but only with a waiver. USA Youth & High School Rugby's High School Eligibility Policy says players must be at least 15 to play high school rugby.

• 14 and enrolled in high school: the player needs the Under 15 Waiver, signed by the player, a parent or guardian, and the coach. Submit it to the player's club and have it ready to show any opponent or administrator who asks.
• No front row: 14-year-olds must not play in the front row in 15s, 10s, 7s or any other contact form of high school rugby.
• Recommended: 14-year-olds should play Junior Varsity or an age-band equivalent such as U16, in line with World Rugby age-banding regulations.
• Middle schoolers: they can't play high school rugby where a middle school (or age-equivalent) competition exists. Where none exists, they may apply to their State Youth Rugby Organization for a case-by-case exemption to play up. The Under 15 Waiver can't be used for middle schoolers.
• Under 14: players younger than 14 must not play high school rugby in any situation.