You may be right. I haven't studied the matter. On reading Lemon, it seems to be based on precedent, at least to a far greater extent then Roe. If it relied on relatively new precedents, well, that's not necessarily the worst thing in the world.
As long as legislatures and school boards do things that touch on the sensitive subject of religion, the courts are going to be called on to see if the state action is Constitutional. These cases can be subtle. The court needs some rules for dealing with the First Amendment. The Lemon test doesn't strike me as wildly outrageous. State action should, for example, have a secular purpose. Who would disagree? The principal or primary effect of a state action must be one that neither advances nor inhibits religion. Again, who would disagree?
The third prong of the Lemon test seems a bit ambiguous -- the state action must not foster "an excessive government entanglement with religion." Lots of room to wiggle around there. Applying it can be tricky. The devil is in the details.
The First Amendment is a good one. It keeps the slimy hands of government out of religious matters. That leaves religion free. Who would disagree with that? I can't imagine how botched up things would be if the government schools, which are entirely incompetent, bloated, corrupt, and generally worthless, started meddling in religious matters.
This entire problem is created by the disagreement about what should be taught in government Schools. Allow true freedom of education and allow each to pick what their children will be taught. The bad schools will die off soon enough.
This will certainly smoke out those who rely upon the government to enforce their particular views. A decentralized government is much less dangerous in most cases.