This dream appears to reflect your feelings and desires surrounding your relationship with this girl, as well as your excitement about your upcoming birthday. Here are some insights into the different elements of your dream:

1. Advanced Math Class: This setting suggests you may feel challenged or stimulated in your current life, particularly in intellectual pursuits. It could also symbolize your desire for growth and learning.
2. Flirting and Connection: The interactions with the girl indicate a deepening emotional connection and attraction. This part of the dream might represent your hopes for a romantic relationship or your desire for companionship.
3. Surprise Visit: Her unexpected arrival could symbolize your longing for spontaneity in your life. It may also reflect your wish for someone to show genuine care and interest in you.
4. Intimacy: The intimate moments you shared, such as smoking together and kissing, suggest a yearning for physical closeness and emotional vulnerability. This part of the dream may indicate a desire to explore deeper connections with others.
5. Birthday Announcement: Her announcing your birthday in class could represent feelings of being appreciated or celebrated, as well as a desire for acknowledgment from peers. It may also symbolize the importance of personal milestones and shared experiences.
6. Spending the Day Together: The idea of spending time together after class signifies hope for future interactions and the potential for your relationship to develop further.

Overall, this dream may be encouraging you to embrace your feelings and take steps towards building a connection with this girl. It suggests that you are open to love and new experiences, especially as you approach a new year in your life. Consider how you might initiate more opportunities to bond with her in waking life.