"your teacher Judy"
This is a sentence where the possessive is "your" so you are interacting directly with the person so probably you should be able to know their gender unless they are one of those people who is really androgynous looking. You mean if you can't tell if the person you are talking to is male or female like Pat on Saturday night live? Usually when I encounter this in real life I am careful about what I say to them and keep dancing around asking them questions until they reveal something that lets me know what gender they are.
However, Arabic in some cases does take the male form as the default option. If you are talking to a crowd of women and men, you would use the male plural form.
E.g.: You are talking to three boys: مُعَلِمُكُمْ
You are talking to three girls: معلمكُنَّ
You are talking to a group of boys and girls: معلمكُمْ
This will probably come up in a later lesson.