"My niece, a high school sophomore, said that her religion teacher told her that in baptism a person is sealed with a special spiritual seal that never goes away? Is this right, and what is this spiritual seal?"
Your niece is correct. Baptism, confirmation, and orders mark the recipient with a spiritual seal that lasts forever. This spiritual seal forever marks the baptized person as joined to Christ. Even if a person leaves the Church or sins mortally, this seal remains on the soul. As a member of Christ’s body, marked with the seal of Christ, the individual receives special helps and graces to do good and avoid evil.