Contract Clause Review

Given the information you've provided, here's an analysis of the clause, "The tenant agrees to vacate the premises within 30 days if the lessor intends to sell the property."
The immediate concern is indeed the instability and unpredictability this clause could bring into your life. As a tenant with children in school, frequent moves can be highly disruptive. The following are some key points of analysis:
Comparatively Short Notice Period: 30 days is a fairly short notice period for a tenant to find new accommodations, especially if you have children and specific requirements (such as proximity to schools). Generally, a longer notice period offers more stability.
Potentially Unenforceable: Depending on the jurisdiction, this clause might be unenforceable. Some locations have laws that protect tenants from sudden eviction due to property sale. It's important to consult a local real estate lawyer to verify this.
Conflict with Other Clauses: The clause stating "The lessor must give 90 days notice before terminating the lease" seems to conflict with the 30-day notice period in this clause. This discrepancy should be clarified.
Possible Negotiation: Since you've mentioned negotiations, you can request an amendment to this clause. One possible approach is to request the clause be modified so that the lease continues even if the property is sold, or at least extend the notice period to align with the 90-day notice clause.
Lack of Fairness: From a fairness perspective, this clause seems to heavily favor the lessor at the cost of tenant stability. This could potentially be argued if this matter were to go to court, though you would need legal advice on this point.
In light of these points, it's recommended to discuss your concerns with your lawyer and negotiate with the lessor to modify this clause or provide additional assurances to prevent the disruption of your living situation. Always consult with a legal professional to understand your rights and potential remedies in your specific jurisdiction.