DS transferred from LCPS to FPCS in 4th grade. We applied at the start of the summer with level 4 parent referral/questionnaire, work samples and most importantly a sealed GT approval packet from LCPS that included (as we were told) teacher recommendations, grade report, test scores and other evaluation criteria for LCPS for GT program acceptance. We also included copies of test scores and grade report just in case the school packet didn't have

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I also knew couple of other kids from DS's class who transferred from LCPS GT to FCPS AAP. I think its less likely to get rejected if your kid coming is from a similar program at a different school system, well unless there is something else (ex: bad grades after GT admission or similar).