Is it permissible to disclose that the ongoing charity (sadaqah jariyah) for the soul of a deceased parent is a contribution towards building a school, with the aim of reminding people of the good deeds of the deceased, or is it better to keep the matter a secret as a benevolent doer?

You can combine informing that the charity is ongoing on behalf of your father, with not disclosing that you are the one who gave the charity, for that is purer for the deed and closer to sincerity. There is no harm in disclosing that you are the one who gave the charity if there is a benefit in doing so.