Arrange review before accepting or starting the placement. If authorization is needed, its route and timing depend on the actual facts; do not promise the organization a start date until those steps and the effective permission are clear. A school deadline does not itself authorize the activity.
Reviewing after starting does not undo the start
An activity that turns out to be unauthorized is not cured by stopping, and the fact of having done it can surface later, including at a future admission or in a subsequent application. Building review time into the plan costs the placement nothing, while a retroactive question can complicate the whole household's position.
A Canadian teacher who chooses the port-of-entry route can apply days before the semester, because there is no petition to await; the risk is that a defective letter is discovered with no time to fix it, so review it in June, not August. A credential assessment is distinct from any required practice licence and the actual immigration permission. Assess the actual activities and required permission before beginning them in the United States. A proposed start date does not resolve an unsettled professional assignment or confer work permission.
Employers often set an effective date for administrative convenience and can move it once they understand why.