Girls Just Want to Have Fun

Janey Glenn is an army brat who is thrilled when her father, Robert, is transferred to Chicago, the home of her favorite dance show Dance TV. At her Catholic girls school, she quickly makes a new friend in Lynne Stone due to their shared love of dance and Dance TV. When Robert nixes the idea of her traveling to downtown Chicago to try out, Janey accompanies Lynne to the auditions.
At the auditions, an enemy is made of spoiled rich girl Natalie Sands when she rudely parks her car where Lynne is walking. The auditions are going well until Lynne's partner gets her cut (it is later found out that Natalie paid him to have her kicked out). Janey and a dreamboat named Jeff both shine, albeit partnered with others at first. They are later paired once the final cut is made. Jeff loves to dance although his father is drilling him to go to trade school and is following his heart instead of his father's wishes.
The two butt heads initially due to their disparate upbringings. Despite Jeff's natural ability to dance, he has never taken a class. Janey has been taking both gymnastics and dance classes for ten years. Helping them to get off on the wrong foot is also Janey's inability to practice when it's convenient for Jeff. Things are further complicated by Natalie's meddling (she finds out that Janey skipped choir practice to meet Jeff and calls her father to tell, posing as "Sister Natalie").
An excellent opportunity for both girls to get back at Natalie presents itself when Jeff is invited to her coming out party. They make copies of her invitation (provided by Jeff's best friend Drew) and pass them out to strangers all over town. Jeff and Drew attend the party and watch the chaos ensue when all of the extra invited people show up (as do Lynne, Janey and Jeff's sister Maggie, watching from the window). Janey and Jeff have become close through their rehearsals. One night, he tells her to meet him not at the rehearsal studio but at a club. While they are enjoying some unstructured dance time, Jeff is taken away by a girl who locked her keys in her car. Meanwhile, a large admirer moves in on her. On Jeff's return, a fight ensues and after Jeff sucker punches the much larger man they run out of the club together. Once at Janey's house, she is aglow over what her life has become: she's in the running to become a Dance TV regular, has a great best friend, as well as a boyfriend. They finally kiss before she excitedly runs inside.
Given the total wreck the party became, the rivalry with Natalie has intensified. She convinces her father to become more involved in insuring her win. This is an easy feat considering that her father owns the company that Jeff's father works for. One day, Natalie's father, J.P. Sands, corners Jeff and tells him that if Natalie doesn't win, his father will lose his job. This puts him in a bad mood and he fights with Janey when he arrives at rehearsal. Her mood quickly matches his when she arrives home and sneaks in the house only to find that her father has installed a security system. He then grounds her for her continuous deception, making it virtually impossible for her to attend the dance contest finals.
Meanwhile, Jeff's surly attitude and decreased desire to be in the contest is noticed by his father. Once he finally gets his son to talk, he simply asks if he can win the contest. When Jeff answers yes he is instructed to do so, job be damned. However, Janey is still on restriction and doesn't know Jeff has changed his mind. However, that changes when her little brother brings her a message that Jeff will compete and she employs Lynne to get her out of the house undetected.
Once Lynne arrives, Janey cuts the wires to the security system and escapes the clutches of the guard dog. When they arrive at the station, Janey makes the elevator up to the studio and Lynne doesn't. She runs upstairs to make the beginning of the show, embracing and kissing Jeff as she arrives. The show begins and the competition is underway. Midway through the show, Janey's family turns to the contest on the television and sees her dancing. Her father, furious, storms out of the house, on the way to the studio. Meanwhile, Janey's mother is in tears with pride as her younger brother cheers his sister on and Jeff's father watches the show from his neighborhood pub.
After all of the dancers are done, the decision comes back: there is a tie between Janey and Jeff and Natalie and her partner. A dance-off ensues. Natalie goes first and when done, strolls off the stage proudly, believing she's won. But after Janey tells Jeff "Let's do it.", they pull out all the stops with a series of synchronized gymnastics Janey has taught Jeff over their time together. When the judges deliberate again, the decision is unanimous: Jeff and Janey win! Natalie is furious and begins to berate her partner over costing her the contest. When she goes to her father to complain, he finally puts his foot down and tells his spoiled child to shut up, to her amazement. When Janey spots her father in the studio, she thinks she's in for trouble, but nothing could be further from the truth as he is extremely proud of her. Miss Dance TV is called to the stage and when she enters, it is none other than Lynne, who has received the job when the former Miss Dance TV quit during the show.

Question: Why does Janey's dad ground her?
Answer: Continuous deception.