The secret behind why Prizmmy and Prism Mates disbanded

Reina's secret

Just as the train stopped at Harajuku station, Karin alighted, squeezing her small body though the hordes of people rushing for work and made her way towards the familiar blue-and-white building which she went to almost every day for 6 years running. It was like a second home to her.

"Good Morning! Sorry I'm late!!" Karin said apologetically as she burst into Prism Box's practice room.
"Onee-chan, you're late!"
Mirei scolded her sister lightly.
"It's okay, you aren't exactly late yet,"
Mia replied with a smile.
"Almost everybody's here except Momo-chan"
"Come to think of it, I haven't seen her all week! I wonder where she went..." Hina said, deep in thought.
"Well," Mia said with a sigh, "We'll just have to start without her,"
The others all shrugged their shoulders and nodded their heads in agreement.

20 minutes into practice, Mia looked at Reina and said,
"Hey Reina, you're awfully quiet today. Is everything alright?" Mia asked with concern in her voice.
Reina took a while to realize that Mia was referring to her.
"Me? Of course everything's fine! Why wouldn't it be?"
Mia looked at her with suspicion. She knew something was definitely wrong.

Later, when Reina excused herself to go to the washroom, Mia trailed her. She was shocked to hear sobbing coming from a cubicle and took a double take. She could hear the muffled sound of Reina's cries.
"I can't do this. I can't tell them that she is going to graduate, I just can't!!"

Mia's eyes widened in shock. Graduate?!!  She thought, horrified by even the word. But, but how and why? Most importantly, who??
Just then Reina emerged out of the cubicle with a tear-stained face. She covered her mouth in shock when she saw Mia standing at the exit.
"Mia?!! What are you doing here?!!"
"I came to check on you. You seemed very....off just now."
Reina lowered her head to hide her tears from Mia. She never thought that she would've let out the secret so easily.
Mia walked up to Reina and put her arms around the girl.
"I never thought that you were going through so much... I'm sorry, Reina..."
At this, Reina burst into tears and buried her head in Mia's shoulder.
"Oh Mia, I'm so so sorry I didn't tell you guys about this earlier. I should've known better as the leader. I really am an idiot..."
Mia took her arms off Reina and looked straight into her eyes.
"No you're not. And I know you're going through a lot right now and I would really appreciate if you would tell me what happened."

"It's about Momona. I think she's going to...graduate"