Such is the political process in our democracy. Nothing illegal about the school board vote, so get 'em at the ballot box. Make it an election issue. Get petitions, walk the blocks getting signatures of like minded people for support. Put your name and few other like-minded names on the ballot for the next election. Run a campaign.
This is how things work in our democracy. It is the worst system, expect for every other government system in the world. Good luck.
NOTE: This answer is made available by the lawyer for educational purposes only. By using or participating in this site you understand that there is no attorney client privilege between you and the attorney responding. This site should not be used as a substitute for competent legal advice from a licensed professional attorney with whom you have established an attorney client relationship and all the privileges that relationship provides. The law changes frequently and varies from jurisdiction to jurisdiction. The information and materials provided are general in nature, and may not apply to a specific factual or legal circumstance described in the question.
Vote them out. My wife and I have problems with our school board and the only option is to organize a campaign against them. If no election is near circulate a petition. And good luck to you.
The political process is best geared to address this issue. Indeed, the issue has been vetted throughout the past 2-3 decades. There is likely no inherent free speech claim; plus, the federal courts have been slowly but surely chopping away at the issue of student speech at the elementary and secondary school level.
A political effort to either unseat or force those elected officials responsible to reconsider is what would need to occur. I perceive that It will be difficult to necessarily put a spin on this showing a cost savings from eliminating the requirement. If anything, uniforms likely further enhance the budget of a school district if the district is receiving some concession from the manufacturer/seller of the items. Absent political change, already a difficult proposition, this will likely prove an uphill battle.