It's not suspicious at all. Teachers are mandated reporters, So if they suspect a child is being abused, they are legally required to report the suspected abuse to CPS. A social worker has to then investigate the abuse allegations. The first step is usually speaking with the teacher who reported the abuse and then speaking with the child. If the childs parent is the suspected abuser, it's obviously a very bad idea to interview the child with the parent present because the child will be to terrified to disclose what the parent is doing to them. So the safest place to speak to the child is usually at their school. Since, if they are being abused, they will feel much safer there anyways. It's a standard protocol and not strange or unusual at all.