2. Be friend with Osana
Your next goal is to be friend with 
Osana before the end of the week. To increase your chance you'll need to buy her student profile in This is the icon you're looking for)
It will cost you 1000 yens or 500 yens if you're in the 
Photography club. Check the Icon in the Saikouphone. Do you have
You can obtain student profiles for free by becoming friend with another student.
You'll need more than 80 Opinion to be friend or 60 with the 
Pink panties equipped. (Available at the Uniqao store with no requirement for 2100 yens or at the pawn shop for 1890 yens.
You can also use the 
Black panties to get +1 Opinion for each social interactions. You can find it with no requirements at the pawn shop for 2250 yens or at the 
Uniqao store after reading the manga 
Chibi Momoko Chan for 2500 yens. Chibi Momoko Chan will cost you 400 yens at the 
book store and 360 yens at the pawn shop. Don't forget to sell the mangas you've done reading at the pawn shop for cash back (10% of the normal price).
You should focus your routine on studying and interacting with students. 
Social science is the key subject to study, by increasing it you'll increase the number of social interactions per day. That means more opinion gained during the day. Study also a bit of 
Language to unlock new mangas.
When having the student profile, youl'l have important details of what the characters likes and dislikes, use this information to your benefit by interacting with the character and choosing the subjects he or she likes. You will be able to use the Compliment interaction to increase your opinion when having done all benefitial topics. If you're in the 
Cooking club and have cookies, you can use them to also increase their opinion.
When you'll have more than 
60 Social science and be friend with 
Osana to unlock the interaction "Ask the rival to reject 
Senpai".
The probability of success will depend on the following parameters:
In your favor: Opinion below 80 and your 
social science stat.
Against you: Police alert level, School Atmosphere, 
Social science of the rival.Osana have 62 in Social science, you'll have at least 60. In the last version you'll have at least 68% to get reject Senpai. (The current version is minimum 84%) This is the easiest way to get rid of your rival in the first week.