您遇到的错误消息与缺少token.json文件有关。这个文件是使用credentials.json文件自动创建的（在您的示例中，根据您的代码片段，我认为它被称为credent.json）。
因此，与在这部分代码中调用credentials.json文件不同：

SCOPES = ['https://www.googleapis.com/auth/classroom.courses.readonly']

creds = Credentials.from_authorized_user_file('credent.json', SCOPES)

您需要调用在授权流完成后创建的令牌文件。authorization flow是下面示例的这一部分：

creds = None
    # The file token.json stores the user's access and refresh tokens, and is
    # created automatically when the authorization flow completes for the first
    # time.
    if os.path.exists('token.json'):
        creds = Credentials.from_authorized_user_file('token.json', SCOPES)
    # If there are no (valid) credentials available, let the user log in.
    if not creds or not creds.valid:
        if creds and creds.expired and creds.refresh_token:
            creds.refresh(Request())
        else:
            flow = InstalledAppFlow.from_client_secrets_file(
                'credentials.json', SCOPES)
            creds = flow.run_local_server(port=0)
        # Save the credentials for the next run
        with open('token.json', 'w') as token:
            token.write(creds.to_json())

一旦代码被创建并存储。您可以像这样创建creds（用于构建服务的凭据）：

SCOPES = ['https://www.googleapis.com/auth/classroom.courses.readonly']

creds = Credentials.from_authorized_user_file('token.json', SCOPES)

• 注意：您可以将凭据文件重命名为credentials.json，也可以将示例代码替换为JSON文件的名称。*

这是一个基于Google文档here的示例代码。
以该样本为基础，它应该是这样的：

from __future__ import print_function

import os.path

from google.auth.transport.requests import Request
from google.oauth2.credentials import Credentials
from google_auth_oauthlib.flow import InstalledAppFlow
from googleapiclient.discovery import build
from googleapiclient.errors import HttpError

# If modifying these scopes, delete the file token.json.
SCOPES = ['https://www.googleapis.com/auth/classroom.courses.readonly']

def main():
    """Shows basic usage of the Classroom API.
    Prints the names of the first 10 courses the user has access to.
    """
    creds = None
    # The file token.json stores the user's access and refresh tokens, and is
    # created automatically when the authorization flow completes for the first
    # time.
    if os.path.exists('token.json'):
        creds = Credentials.from_authorized_user_file('token.json', SCOPES)
    # If there are no (valid) credentials available, let the user log in.
    if not creds or not creds.valid:
        if creds and creds.expired and creds.refresh_token:
            creds.refresh(Request())
        else:
            flow = InstalledAppFlow.from_client_secrets_file(
                'credentials.json', SCOPES)
            creds = flow.run_local_server(port=0)
        # Save the credentials for the next run
        with open('token.json', 'w') as token:
            token.write(creds.to_json())

    try:
        service = build('classroom', 'v1', credentials=creds)

        # Here you will add whatever method you want to use. 

    except HttpError as error:
        print('An error occurred: %s' % error)

if __name__ == '__main__':
    main()

参考：

• Google Documentation "Python quickstart"
• 获取OAuth2刷新令牌并配置您的客户端