A.S.
My daughter goes to Beaver Technology Center in GISD. We had her evaluated the February before Kindergarten. All kids are tested on the same basis for the Gifted/Talented, Magnet, and Classical program. They are given a percentage grade and if they fit into a certain bracket that's where they go. If they qualify for GT then the top kids that fill the slots will get in within the bus zones. For instance: If you live in a bus zone for Walnut Glen Academy and they take 40 students and your child placed #39 then they have a slot. For Magnet Schools - it is a lottery. Your child qualifies for a school within your bus zone and they need to fill 40 slots. They throw all the names into a hat and then draw 40 - your child doesn't get chosen and then placed on a waiting list.
The testing is done one on one with an evaluator and the positions are highly competitive. There is actually two rounds of testing for admittance into the program and they evaluate on 15 different criteria - imagination, knowledge, initiative, willingness to learn - the potential for GT essentially. If you turn in an application early all it does is get you a testing slot you like vs one you accept. All the school do some selection based on ethnicity but there is nothing obvious that tells how.
All the schools accept "neighborhood" kids. The GT schools have separate classrooms. The magnet schools incorporate them into the program.
You can PM me with other questions if you have them. You did the right thing by looking early. Informational meetings usually take place in December/January.