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Hello.
Columbia Law requires a dean certification from the current law school in order to transfer. When is a good time to ask for a Dean certification? Will an academic dean suffice, or should it be the dean from the whole law school?
Also, any advice about asking would be helpful.

My law school has a class size of ~300 so I'm not sure the dean knows me. Will that hurt me in the application process?

You can just ask your registrar's office to do this. It's probably not even a dean that fills it out. An assistant will probably do this. It's just a form they fill out. They'll just pull your record to fill it it out. No one needs to know who you are, it won't hurt you at all.

Depending on when you want to submit your application, you should ask about a month in advance to give the office time to process. Consider whether the registrar's office will be busy processing grades/transcripts/other requests at the time too. They won't care that you are asking for this, just be polite when you ask.