With the debate over gun violence reshaped by the shooting deaths of 20 children and six adults at a Connecticut elementary school last month, lawmakers across the country are pushing proposals to arm teachers in the classroom. But many of them may be wasting their time.

More than a third of the states already allow teachers and other adults to carry guns to school. In most cases, all you need is the equivalent of a note from the principal — you usually don't even need law enforcement approval.

NBC News reviewed the firearms and education laws in all 50 states and found that 18 of them allow adults to have a loaded gun on school grounds, usually as long as they have written permission.

That's for pretty much any reason; the list doesn't include states that generally ban guns but carve out narrow exceptions for specific activities like safety demonstrations or military formations and parades.

During a Tea Party forum in Fort Worth, Texas Gov. Rick Perry became one of the first prominent officials after the Dec. 14 shootings to call for teachers to be allowed to carry firearms to work — even though Texas already allows any qualified adult to do so as long as the principal OKs it.

Since then, lawmakers in several states have jumped on board with proposals that mirror laws already on the books.

In Alabama, for example, state Rep. Kerry Rich, a Republican representing DeKalb and Marshall counties, has proposed legislation that would give schools the option of letting their teachers or administrators carry guns.

They already have that option.

In Kentucky, lawmakers said last week they're contemplating  legislation that would let teachers and administrators pack a weapon after completing safety training.

Kentucky school boards already have the authority to allow that.

Oregon state Rep. Dennis Richardson, a Republican representing Central Point, said in an e-mail to schools superintendents in his district that, he too, supported allowing teachers to carry weapons at school.

Oregon school boards can already give that permission.

Self defense
The real debate is over proposals to create exemptions from gun bans in states that don't have them. Such measures have been introduced or proposed in Alaska, Florida, Indiana, Maine, Minnesota, Missouri, Nevada, Oklahoma, Pennsylvania, South Carolina, South Dakota and Tennessee. (The Michigan Legislature passed a bill last year, but Gov. Rick Snyder vetoed it.)

"Parents will not let politicians stand and do nothing," said Tennessee state Rep. Eric Watson, a Republican representing Bradley County who is one of several Tennessee lawmakers planning to introduce legislation to allow properly certified school district employees to arm themselves.

Bradley County already has armed deputies, known as school resource officers, or SROs, in all 17 of its schools. But Watson said that's not enough.

"I'm not trying to take the position of an SRO. This is in support of an SRO," Watson, a former sheriff's captain, told NBC station WRCB of Chattanooga.

The American Federation of Teachers and the National Education Association have both campaigned against such measures, but not all local school administrators share their position.

In Tennessee, the membership of Professional Educators of Tennessee is "split about 50-50," on proposals like Watson's, said J.C. Bowman, a spokesman for the organization.

"Some don't want the responsibility, and they worry about liability," Bowman said. "But this doesn't prohibit it and it doesn't require it, so that's something we can work with."

Jim Rigano, a member of the Springboro, Ohio, school board, said he hoped to craft a policy to allow school employees with proper state permits to carry guns.

"I think it's about giving employees the opportunity to defend themselves," he told NBC station WDTN of Dayton.

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President Barack Obama called for more, not less, gun control after last month's Connecticut massacre. But there's little the federal government can do in states that pass measures relaxing school regulations.

The Gun-Free School Zones Act of 1990 prohibits anyone from having a firearm in a school zone. But that law includes the same exception recognized by the states identified in NBC News' survey: It doesn't apply if the weapons are "approved by a school in the school zone."

And in any event, Reps. Thomas Massie, R-Ky., and Steve Stockman, R-Texas, have introduced measures in the new Congress that would repeal the federal law altogether.

Here are the 18 states that allow adults to carry loaded weapons onto school grounds with few or minor conditions:

• Alabama (which bans possessing a weapon on school grounds only if the carrier has "intent to do bodily harm")
• California (with approval of the superintendent)
• Connecticut (with approval of "school officials")
• Hawaii (no specific law)
• Idaho (with school trustees' approval)
• Iowa (with "authorization")
• Kentucky (with school board approval)
• Massachusetts (with approval of the school board or principal)
• Mississippi (with school board approval)
• Montana (with school trustees' permission)
• New Hampshire (ban applies only to pupils, not adults)
• New Jersey (with approval from the school's "governing officer")
• New York (with the school's approval)
• Oregon (with school board approval)
• Rhode Island (with a state concealed weapons permit)
• Texas (with the school's permission)
• Utah (with approval of the "responsible school administrator")
• Wyoming (as long as it's not concealed)

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