Hi @sobaidullahali ,
Suggestion: The simplest and easiest way to achieve this using OOB features is to change your school type (choice) column to a lookup. By doing this you can enable OOB lookup filtration by using Lookup Related records properties. Also, if you have to show your school type (lookup) column as drop down on Portal you can change it using OOB Portal feature to show your lookup as list. By doing this you will have flexibility to add more schools anytime and you don't have to do any changes even if you change any relationship.
On other hand, if you still want to still to your current model and dropping column is not an option then have to add custom script to achieve this, there is no OOB workaround available on Portal side. You check following article to achieve this:
https://oliverrodrigues365.com/2021/01/24/power-apps-portals-custom-lookup-filter/
Feel free to ask, if you have any questions.
Thanks,
Saud
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