"Why isn't prayer and religious teaching allowed in our public schools?"
Allowed should be distinguished with mandatory. Religious teaching is normally available but is ideally non-confessional, where one learns about the various great religions, their origins, main myths and tenets. Everyone is allowed to pray, but it is not institutionalized, the latter would force it on unwilling participants or on people of other religions, which is unethical and a violation of their rights.
"No wonder our schools have been going down the tube in the last 30 years...obviously due to the absence of prayer."
Oh, the good old days... Unsubstantiated claim. But in parts of the US, the quality of education is still at stake. Even today, some fundamentalists are attacking the school system in attempt to dictate what should or should not be taught, disregarding the consensus of expert pedagogues, etc.
"As a taxpayer, I DEMAND our children be allowed to both PRAY and BE TAUGHT THE GOOD NEWS OF JESUS in our public schools."
What you want is not them being "allowed" but for it to be forced unto them. As others have noted, unless it's homeschooling or a confessional private school, the separation of church and state prevails, per your constitution. Everybody pays taxes and that does not give them the right to violate that.
"Also, it would be good if the 10 commandments were posted in classrooms, as to avoid all the school shootings. People will get discouraged once they learn "Thou shall not kill" is on there. So why is this behavior being encouraged by the absence of these commandments?"
As some noted before me, another fallacy. Noone encourages killings by not corrupting the school system in favor of a particular religion. Frivolous accusation. Murder is already illegal under law anyway. Ironically, if more people understood that, there would be less conflicts.