The best thing you can do as a parent is to support your child. It is important that the goal of the Gymnasium is really your son's or daughter's dream and not yours. The entrance exam and the following time at the short high school are very demanding and can only be mastered if there is enough motivation of your own. So make sure that you do not push your child to do anything.
It can be very helpful to create a schedule with your child. Find out about the content of the exam, narrow down topics, and create a schedule that specifies by when which topic should be learned and when it should be practiced. Constant repetition of what you have already learned is enormously important. Ideally, the schedule should end at least one month before the entrance exam so that there is still enough time as a buffer. Free times in which you do not study or practice should also be a fixed part of the schedule and should be adhered to. Feel free to alternate the form of learning (e.g. reading, quizzing, with flashcards, solving independently, just listening, concentrated alone, under time pressure, together with others, playful, exam simulations, etc.).
Another important point is that you should create several folders with your child, in which you systematically store documents. Organize the folders according to subject, topic, theory, exercise, solutions, etc.. This ensures that documents can be found quickly and that the overview is maintained.
If you notice that your child is not keeping up or is having increased difficulties, then it makes sense to seek professional help. Either in the form of private tutoring or a preparatory course. The advantage of private tutoring is that it can be tailored to your child's individual needs. Preparatory courses, on the other hand, are highly specialized in grammar school preparation and can optimally work towards the entrance exam.
It is also beneficial if your child has a balance to learning. Be it a sport, a club or time with family and friends. Simply something that is fun and has nothing to do with gym preparation, to get other thoughts and to be able to switch off. Fresh air, enough sleep and a wholesome, balanced diet certainly also play a role in ensuring that your child has enough energy.
Keep in mind that not every child is suited for high school. Perhaps your child is more suited to something practical. Listen carefully to your child and never put him or her under pressure. Of course it will be tough; difficulties and low motivation are perfectly normal. Encourage your child to stick with it and keep going. Constructive criticism and praise help. The joy of learning and curiosity should never be lost. Students who really want to go to a short high school will have the necessary motivation, ambition and willingness to learn by themselves. You as parents can only be supportive, but your child must master the entrance examination and the later everyday life at the Gymnasium on his or her own.