Teaching Creationism in a public school is not a violation of the Establishment Clause. If a school, knowing its business of education, decides that it should be taught in the curriculum, who's to object?It should be, however, taught under the topic of Religion, and not Science.
Creationism, or Intelligent Design and Evolution are not competing philosophies; one has been proven as fact through the Scientific Method, the other is based on Biblical interpretation of "fact." One is not interchangeable with the other, and each should remain within its respective category and treated as such.
The other question raised is why a state legislature has the power to dictate a public school's course of study. This appears to be the more dangerous precedent.
Whether it is a violation of the establishment clause is a matter of political opinion.
To liberals, it clearly is a violation. They would argue that the only reason for teaching creationism is to advance a specifically religious world view. Using state schools to do this would clearly violate the establishment clause in their minds.
Social conservatives do not see it this way. They feel that creationism/intelligent design is a scientifically sound alternative hypothesis to evolution. They argue that evolution is a hypothesis and not a fact. By this logic, teaching creationism is not based on the desire to advance religion. It is, instead, based on the desire to teach a possible explanation that is accepted (in their minds) by many scientists. They might also argue that the establishment clause only refers to a government actually establishing an official religion, not to things like teaching creation (or prayer in schools) that simply supports religion.
So, the answer to this question is based largely on a person's political opinions.