More than 17,000 Leaving Certificate students have registered their details in an online registration system for this year's calculated grades.
Registration opened at 10am and will remain open until 10pm on Thursday.
All of the country’s 61,000 Leaving Certificate students are required to register via this new online portal established by a special unit within the Department of Education.
Students need the exam number already given to them as well as their PPS number and contact details.
They will then be given the opportunity to opt in to and confirm the level at which they intend to be examined in each of their subjects.
Students are also required to confirm the level at which they intend to sit each of their subjects, and if they wish they can change to a lower level.
Reports are that it is a quick process with one student had it completed in three-and-a-half minutes.
Students are being urged to register early rather than leave it until the last minute.
The grading system is being put in place because it was felt the health risks to students posed by Covid-19 were too great to hold the Leaving Certificate examinations in the normal way.
Speaking on RTÉ's Today with Sarah McInerney, school principal Colm O'Rourke said he has "concerns" for students who do not access the system.
He called on the Department of Education to ensure that students are contacted if they have not logged on to the portal in the coming days.
He said he hoped there was a mechanism that allowed the department to become aware of students who have not registered within the adequate timeframe and that their schools can make contact with them.
Mr O'Rourke said: "There are people slightly outside the system, that maybe don't care that much, maybe they don't hear or understand what is going on, and they won't be included in the final numbers."
The Department of Education said it is essential that students complete the online process to enable their schools to provide information on their expected level of achievement in each subject.
More than 60,000 students are expected to register.
The address is gov.ie/leavingcertificate.
Mr O'Rourke said that when completing the online process students should consider whether they would have passed the particular subject at the same level that they opted for in January.
"If not, you should go down a level from Higher to Ordinary or Ordinary to Foundation," he said.
This will be the students' only opportunity to make this decision, Mr O'Rourke said.
"It's all duck or no dinner at this stage," he added.
He said he wanted to reassure parents and students that while schools may have had prior concerns about the calculated grade system, teachers will "get on and make the best of it now".
He said: "We all know there are flaws, and we could complain, and there are many unknowns, but parents and students should understand that we're going to do this as best as we can and it will work out for the vast majority of people."
Leaving Cert student Daniel Cremin said completing the online process was "easy".
In relation to the uncertainty surrounding the Leaving Cert exams, he described as "stressful" the lack of information on the exams process and the recent uncertainty about the involvement of members of the Association of Secondary Teachers Ireland.
He said he is reassured that members of the State Examinations Commission are included on the advisory group to the department.
The Department of Education said every school has been sent information on the portal, and how to register, and will be asked to circulate the information to students and to parents and guardians.
Schools have been asked to assist any student who may need help getting access online.
The Department said there will be helpline support from midday today.